with reference to fig1 , a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis x - x . the direction x - x may be referred to as the axial direction of the engine . the engine 10 comprises , in axial flow series , an air intake 11 , a propulsive fan 12 , an intermediate pressure compressor 13 , a high - pressure compressor 14 , combustion equipment 15 , a high - pressure turbine 16 , an intermediate pressure turbine 17 , a low - pressure turbine 18 and a core engine exhaust nozzle 19 . a nacelle generally surrounds the engine 10 and defines the intake 11 , a bypass duct 22 and a bypass exhaust nozzle 23 . the gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows : a first air flow a into the intermediate pressure compressor 13 and a second air flow b ( which may be referred to as a bypass flow b ) which passes through the bypass duct 22 to provide propulsive thrust . the intermediate pressure compressor 13 compresses the air flow a directed into it before delivering that air to the high pressure compressor 14 where further compression takes place . the compressed air exhausted from the high - pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted . the resultant hot combustion products then expand through , and thereby drive the high , intermediate and low - pressure turbines 16 , 17 , 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust . the high , intermediate and low - pressure turbines 16 , 17 , 18 respectively drive the high and intermediate pressure compressors 14 , 13 and the fan 12 by suitable interconnecting shafts . as the air passes through the gas turbine engine 10 it is heated to high temperatures . in particular , the first airflow a reaches high temperatures as it passes through the core of the engine . typically , particularly high temperatures may be reached at the exit of the combustion equipment 15 , and as the air subsequently passes through the high , intermediate and low - pressure turbines 16 , 17 , 18 . the gas turbine engine 10 comprises outlet guide vanes ( ogvs ) 100 extending across the bypass duct 22 , which therefore sit in the bypass flow b . each ogv 100 takes the form of a large stator vane , and thus may be referred to as an aerofoil or aerofoil component 100 . a plurality of ogvs 100 is typically provided as an annular array in the bypass duct 22 . because each ogv 100 is an especially large aerofoil component , it is particularly advantageous to reduce its weight . thus , the ogv 100 in the gas turbine engine 10 is not a solid , homogeneous structure , but instead has a hollow cavity filled with a lighter weight material . fig2 is a schematic of an aerofoil having a filled cavity ( which may be referred to herein as a filled aerofoil ) in accordance with the invention . in the fig2 example , the filled aerofoil is a filled outlet guide vane ( ogv ) 100 . the filled ogv 100 is manufactured by joining a capping panel 200 to a pocketed aerofoil body 300 . the capping panel 200 and the pocketed aerofoil body 300 may be any suitable material , for example metallic and / or composite , and may be the same or different materials . in fig2 , the interface between the pocketed aerofoil body 300 and the capping panel 200 is indicated by the line 110 . however , it will be appreciated that after manufacture of the filled aerofoil 100 , the join between the original capping panel 200 and the pocketed aerofoil body 300 will be seamless , and may not be visible . as such , the line 110 in fig1 is shown merely for the purpose of indicating the original interface between the capping panel 200 and the pocketed aerofoil body 300 prior to the joining and optional finishing of the filled aerofoil 100 . furthermore , it will be appreciated that the extent of the capping panel 200 is not limited to that illustrated by the line 110 in fig2 , which is merely illustrative . in the example shown in fig2 , the filled ogv 100 comprises an inner attachment 330 ( which may be a platform ) and an outer attachment 340 , which may be used to attach the finished ogv 100 to the rest of the gas turbine engine . such inner and outer attachments 330 , 340 may be a part of the pocketed aerofoil body 300 . however , it will be appreciated that some pocketed aerofoil bodies 300 may not include one or both of the inner attachment 330 and outer attachment 340 . for example , the inner and / or outer attachments 330 / 340 may be separate features that may be attached to the filled aerofoil 100 after the capping panel 200 and the pocketed aerofoil body 300 have been joined together . fig3 shows a close - up cross sectional view of the capping panel 200 , the pocketed aerofoil body 300 , and a pre - formed filler insert 400 during manufacture of the filled ogv 100 . it will be appreciated that fig3 shows only a part of the capping panel 200 , pocketed aerofoil body 300 , and pre - formed filler insert 400 ( which may be referred to herein as an insert 400 ), and that the full parts extend in the chordwise direction , to the left in fig3 .′ as shown in fig3 , the pocketed aerofoil body 300 comprises a pocket 310 which , prior to being covered by the capping panel 200 , is an open pocket 310 . the pocket 310 is formed in , and surrounded by , a surrounding hollowed surface 320 . in all examples , the surrounding hollowed surface 320 may have a step ( which may be a recess , or pocket ) 325 formed therein ( for example around at least a part of the perimeter of the pocket 310 ), as in the fig3 example . the step 325 may support the capping panel 200 during manufacture , again as shown in the fig3 example . the step may or may not support the capping panel 200 in the capping position , because the capping position may be defined by the insert 400 , optionally in combination with a tool 500 . however , other examples , which may otherwise be substantially as shown in fig3 , may not have such a step 325 . where no step 325 is provided , the capping panel 200 may be supported in any other suitable manner , for example using a different physical support , or by providing a low pressure ( such as a vacuum or pseudo - vacuum ) over the outer surface of the capping panel 200 . in other arrangements , such as illustrated in fig4 below , the capping panel 200 may be supported directly by the insert 400 . during manufacture , the insert 400 is inserted ( or positioned ) in the cavity 310 . the insert 400 may also be referred to as a core 400 . in the fig3 example , there is a gap between the insert 400 and an inner surface 210 of the capping panel 200 . in other words , in the fig3 example , the insert 400 does not engage with , or support , the capping panel 200 when the components are initially put into position . this means that , in the fig3 example , the depth a of the pocket 310 is greater than the combined thickness ( b + c ) of the capping panel 200 and the insert 400 when they are initially put into position for manufacture . the insert 400 may be an expandable insert 400 , which may be referred to as a foaming insert 400 . such a foaming insert 400 may comprise and / or be formed as described elsewhere herein . such a foaming insert 400 may be activated under given conditions , for example under given conditions of temperature and / or pressure and / or by providing an activation agent , such as a chemical activation agent , as prescribed for the particular material . in the fig3 example , once activated the foaming insert 400 comes into contact with the capping panel 200 , in particular with the inner surface 210 of the capping panel 200 . the foaming insert 400 may then substantially fill the cavity 310 ( not shown in the figures ). under activation , the foaming insert 400 may exert a force on the capping panel 200 as it tries to expand further . the capping panel 200 may be constrained from moving away from the cavity 310 , for example by appropriate tooling 500 . the tooling 500 may have an engagement surface 520 that has the shape of the desired aerofoil profile over a portion at which it engages at least with an outer surface 220 of the capping panel 200 . as such , if the capping panel is provided with a force away from the cavity 320 , for example under the action of a foaming insert 400 , it is pressed into the engaging surface 520 of the tooling 500 . this may help to form an accurate aerodynamic profile over this region . this results in an accurate gas - washed profile of the finished aerofoil . activation ( or curing ) of the foaming insert 400 may result in it being bonded to the capping panel 200 , and it may therefore act to hold the capping panel 200 to the pocketed aerofoil body 300 in the desired relative position . as described in more detail elsewhere herein , the capping panel 200 and the pocketed aerofoil body 300 may then be permanently bonded or joined together , for example by welding . the foaming insert 400 may have a barrier layer 410 formed around its periphery , as in the fig3 example . the barrier layer 410 may be in the form of a tape , for example . the barrier layer 410 may be , for example , a polymer and / or a metallic barrier layer 410 . such a barrier layer 410 may be arranged to prevent any material from the insert 400 ( or any associated adhesive ) entering and / or contaminating the joint or bond ( for example weld ) between the capping panel 200 and the pocketed aerofoil body 300 . such a barrier layer may , of course , be provided to any insert 400 according to the invention . fig4 shows a filled capping panel 400 during manufacture using an alternative process . in the arrangements shown in fig3 and 4 , like features are provided with like reference numerals . in fig4 , the insert 400 engages with , and thus supports , the inner surface 210 of the capping panel 200 throughout manufacture . the insert 400 shown in fig4 may be a precision engineered insert 400 to ensure that the capping panel 200 , and particularly the outer surface 220 of the capping panel 200 , is located accurately in the desired position when located on the insert 400 . in contrast to the fig3 arrangement , no step 325 in the pocketed aerofoil body 300 is required in order to provide initial support to the capping panel 200 , which is instead positioned and supported on the insert 400 throughout . any suitable material may be used for the insert 400 shown in fig4 , such as a composite or a homogeneous material . typically , the insert 400 would not be a foaming insert such as that shown in fig3 , but would instead retain its size and shape throughout manufacture . however , the insert 400 of the fig4 arrangement may be a foaming insert , in which case a tool such as the tool 500 shown in fig3 , may be provided to constrain the capping panel 200 when the insert 400 is activated . the insert 400 may be substantially incompressible throughout the joining process . for example , the insert 400 may be substantially incompressible even when subjected to elevated pressure and / or temperature which may be used to activate an adhesive that may be used to bond the capping panel 200 to the insert 400 . as an alternative to being substantially incompressible , the insert 400 may experience a degree of compression , expansion , or deformation during the manufacture of the filled aerofoil 100 , but in that case the compression / expansion / deformation would be specifically designed to ensure that the capping panel 200 is retained in the desired position during subsequent joining ( for example welding , as described elsewhere herein ). once the capping panel 200 has been positioned on the insert 400 , it may be held in position , for example using an adhesive , such as a film adhesive 420 . as with the fig3 example , a barrier material 410 may be provided to surround the insert 400 . fig5 shows a section through a part of a completed filled aerofoil 100 . the filled aerofoil 100 shown in fig5 may be manufactured according to the invention , such as by the methods and apparatus shown and described in relation to either fig3 or fig4 . fig5 shows a permanent bond or joint in the form of a weld 600 between the capping panel 200 and the pocketed aerofoil body 300 . the weld 600 may be formed by any suitable welding process . for example , the weld 600 may be a laser weld 600 . the weld 600 may , for example , extend around the interface between the capping panel 200 and the pocketed aerofoil body 300 , such as that illustrated by way of example in fig2 by the line 110 . because the capping panel is supported by the insert 400 in the interim state prior to welding , it may not be necessary to clamp the capping panel 200 during welding . this may be particularly advantageous where no step 325 is provided in the pocketed aerofoil body 300 , such as in the example shown in fig4 . typically , immediately after a weld is formed , the area around the weld tends to shrink as it cools . this can cause problems with distortion of the geometry . however , because the capping panel 200 is supported over a significant portion of the area of its inner surface 210 ( for example substantially all of its area ) according to the invention , the conventional problems associated with post - weld distortion can be avoided , or at least reduced . instead of distorting , the capping panel 200 used in the present invention would tend to adopt a ( desired ) shape corresponding to the underlying insert 400 . in turn , this may allow a thinner capping panel 200 than would conventionally be required to be used , because a reduced tolerance for post - weld machining may be used . as such , the shape of the capping panel 200 may be defined at least in part by the insert 400 on which it is supported during welding . as such , the insert 400 may at least in part define the shape of the capping panel 200 , for example the shape of the inner surface 210 and / or the outer surface 220 of the capping panel 200 . in the examples described herein , the capping panel 200 forms a part of the suction surface 120 of the finished filled aerofoil 100 . however , it will be appreciated that the capping panel could form a part of the pressure surface 130 . the capping panel 200 , the weld 600 , and the surrounding hollowed surface 320 of the pocketed aerofoil body 300 may all form part of the suction surface 120 ( as shown in the fig5 example ) or the pressure surface . a finishing process , such as machining or linishing , may be used after forming the permanent joint 600 between the capping panel 200 and the pocketed aerofoil black 600 , for example to avoid any unwanted material remaining after the joining step . it will be appreciated that it could be applied to any aerofoil or aerofoil component , such as any rotor blade or stator vane , for example for use in a turbine , compressor , or other aerofoil - shaped component of a gas turbine engine . purely by way of example , the aerofoil 100 may be a filled ogv 100 such as that shown by way of example in fig1 and 2 . it will be appreciated that many designs and / or arrangements of features , such as capping panel 200 , pocketed aerofoil body 300 or insert 400 , other than those shown in and described in relation to fig1 to 5 and not explicitly described herein fall within the scope of the invention . furthermore , any feature described and / or claimed herein may be combined with any other compatible feature described in relation to the same or another embodiment .