referring to the drawings , the various preferred embodiments of the invention will now be described . the same numbering will be used to indicate the same or similar features of each of the embodiments . [ 0067 ] fig1 shows a linkage mechanism 5 arranged between a fixed member 6 and a movable support 7 . the linkage mechanism 5 in this embodiment comprises a first bowed linkage arm 8 pivotally attached at one end 9 to a bracket 10 fixed to the member 6 and pivotally attached at a second end 11 to the movable support 7 . a second linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and above the connection point 15 of the first linkage arm 8 and at its other end 16 to a connector pin 17 . a third linkage arm 18 is pivotally attached at one end 19 to the movable support 7 at a connection point 20 spaced from and above the connection point 21 of the first linkage arm 8 and at its other end 22 to the connector pin 17 . the connector pin 17 links the respective ends 16 and 22 of the second and third linkage arms to a point 23 intermediate the ends of the first linkage arm 8 . connector pin 17 passes through the ends 16 and 22 and is slideably received in a slot 24 . the movable support may be raised and lowered between a first and second position defined by the limits of movement of connector pin 17 within slot 24 . each of the pivotal attachments described can be considered fixed in they typically consist of a fixed pin or bush mating with a complementary hole to provide pivotal movement about a fixed axis of the pin . in alternative embodiments , the attachment points may be provided by a movable pivot , typically a pin or bush slideably mating with a slot though other functionally equivalent mechanisms could be used . [ 0069 ] fig2 shows a second embodiment of the first aspect of the present invention . the linkage mechanism 5 in this embodiment comprises a first linkage arm 8 pivotally attached at one end 9 to a bracket 10 fixed to the member 6 and pivotally attached at a second end 11 to the movable support 7 . a second linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and above the connection point 15 of the first linkage arm 8 and at its other end 16 to a connector pin 17 . a third linkage arm 18 is pivotally attached at one end 19 to the movable support 7 at a connection point 20 spaced from and above the connection point 21 of the first linkage arm 8 and at its other end 22 to a connection point 30 intermediate the ends of the second linkage arm 12 . the connector pin 17 links the respective ends 16 and 22 of the second and third linkage arms to a point 23 intermediate the ends of the first linkage arm 8 . connector pin 17 passes through the ends 16 and 22 and is slideably received in a slot 24 . [ 0070 ] fig3 shows an embodiment of the second aspect of the invention . the figure illustrates the first and second positions of the movable support 7 and the corresponding movement of the linkage mechanism 5 . the linkage mechanism 5 in this embodiment comprises a first linkage arm 8 which is reverse - bowed about a central point and pivotally attached at one end 9 to a bracket 10 and pivotally attached at a second end 11 to the movable support 7 . a second linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and above and in front of the connection point 15 of the first linkage arm 8 and at its other end 16 to a connector pin 17 . a third linkage arm 18 is pivotally attached at one end 19 to the movable support 7 at a connection point 20 spaced from and above the connection point 21 of the first linkage arm 8 and at its other end 22 to the connector pin 17 . the connector pin 17 links the respective ends 16 and 22 of the second and third linkage arms to a point 23 intermediate the ends of the first linkage arm 8 . connector pin 17 passes through the ends 16 and 22 and is slideably received in a slot 24 . in this embodiment the movable support 7 includes a platform or surface 35 for supporting a load . again , as shown in the figures the movable support 7 may be raised and lowered between a first and second position defined by the limits of movement of connector pin 17 within slot 24 . [ 0071 ] fig4 and 5 illustrate the third aspect of the invention and are shown employing a linkage 5 according to the second aspect though it will appreciated other forms of linkage may be used . referring in particular to fig6 the is movable support 7 is generally l - shaped to provide a support platform 35 . a pair of spaced guide formations in the form of slots 36 and 37 are provided respectively adjacent the apex 38 of the platform 35 and the distal end 39 of the support platform 35 . an adjustable platen 40 having a pair of arms 41 and 42 each of which are provided with a respective control formation 43 and 44 extending normal to the longitudinal axis of the arms . the control formations 43 and 44 are in the form of bushes or pins receivable in an associated guide slot 36 and 37 . the slots and arms are configured relative to one another such that movement of the control formations between the extremities of their associated slots results in movement of the platen 40 between a first and second position , the change in position further resulting in a change of inclination of the platen 40 as illustrated in dotted outline in fig6 . the inclination of the guide slots 36 and 37 relative to each other provides for an automatic locking force to be applied to the platen 40 when located at positions intermediate the first and second positions . [ 0072 ] fig6 illustrates an embodiment of a fourth aspect of the present invention . the embodiment is similar to that of fig5 but does not require the linkage and uses a fixed support bracket 35 ′ in the form of the support platform 35 . the control of the platen 40 operates in a similar manner and the slots 36 and 37 are inclined relative to each other so that the platen is gravitationally biased into locking engagement with the base bracket 35 ′. translational movement of the platen 40 under a force applied by the user can move it to a second position as illustrated in dotted outline . once the force is removed the platen 40 will return to the locked position . fig7 to 9 illustrate a further embodiment of the present invention . the 5 linkage mechanism 5 in this embodiment comprises a first bowed linkage arm 8 pivotally attached at one end 9 to a bracket 10 and pivotally attached at its other end 11 to the movable support 7 . a second bowed linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and above the connection point 15 of the first linkage arm 8 and at its other end 16 to the movable support 7 at an attachment point 20 . the first linkage arm 8 is pivotally attached at its end 11 to an attachment point 45 which is in the form of a slot 46 which slideably receives a bush or pin 47 to provide a movable pivotable attachment . attachment point 45 is spaced from and below the connection point 20 of the second linkage arm . the second linkage arm 12 is provided with a slot 24 for slideably receiving a connector pin 17 extending normally from the first linkage arm 8 so as to maintain , in conjunction with movable pivot 45 , a constant attitude of the movable support . [ 0074 ] fig1 , 11a , 11 b , 12 a and 12 b illustrate a further embodiment of the present invention and in particular a locking mechanism for the linkage mechanism . the linkage mechanism 5 in this embodiment comprises a first bowed linkage arm 8 pivotally attached at one end 9 to a bracket 10 fixed to the member 6 and pivotally attached at a second end 11 to the movable support 7 . a second linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and below the connection point 15 of the first linkage arm 8 and at its other end 16 to a connector pin 17 . a third linkage arm 18 is pivotally attached at one end 19 to the movable support 7 at a connection point 20 spaced from and below the connection point 21 of the first linkage arm 8 and at its other end 22 to the connector pin 17 . the connector pin 17 links the respective ends 16 and 22 of the second and third linkage arms to a point 23 intermediate the ends of the first linkage arm 8 . connector pin 17 passes through the ends 16 and 22 and is slideably received in a slot 24 . the pivotal attachment at end 9 of the linkage arm 8 is a movable pivot comprising a horizontal slot 50 in the end 9 slideably receiving a pin or bush 49 extending from the bracket 10 . this enables translational to and fro movement of the movable support 7 and associated linkage 5 relative to the bracket 10 and fixed member 6 . the translational movement being between a locked position and an unlocked position . the bracket 10 is provided with an arcuate slot 51 slideably receiving a locking key 52 which is pivotally attached to linkage arm 8 . the locking key 52 shown in detail in fig1 has a tapered groove 53 in one side 54 which slideably receives one edge 55 of the slot 51 . in said unlocked position the locking key 52 slides freely along the slot edge 55 , however , when linkage is moved horizontally by the user to a locked position , the key groove engages with the edge of the slot 55 to lock the linkage mechanism in any desired position between the normal extremities of movement set by slot 24 . a further embodiment of the first aspect of the invention is illustrated in fig1 . the linkage mechanism 60 in this embodiment comprises a first linkage arm 8 pivotally attached at one end 9 to a bracket 10 fixed to the member 6 . a second linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and above the connection point 15 of the first linkage arm 8 and at its other end 16 to a connector pin 17 . a third linkage arm 18 is pivotally attached at one end 19 to the movable support 7 at a connection point 20 and at its other end 22 to a fourth linkage arm 61 at one end 62 . a further connector pin 63 is provided at the other end 64 of the fourth linkage arm 61 . the other end 11 of first linkage arm 8 is pivotally attached to one end 65 of a first link 66 . the other end 67 of the first link 66 is provided with a connector pin 68 which pivotally connects to one end 69 of a second link 70 and is slideably received in a slot 71 provided intermediate the ends 19 and 22 of the third linkage arm 18 . the other end 72 of the second link 70 is pivotally attached to the movable support 7 at a connection point 21 spaced from and below connection point 20 . a third link 76 connects the pivotal connection point 77 of third and fourth linkage arms 18 , 61 and the pivotal connection point 78 of the first linkage arm 8 and the first link 66 . the connector pin 17 links end 16 of the second linkage arm to a point 23 intermediate the ends of the first linkage arm 8 . connector pin 17 passes through the end 16 and is slideably received in a slot 24 formed in linkage arm . the connector pin 63 links end 64 of of the fourth linkage arm to a point 73 spaced from and adjacent point 23 . connector pin 63 passes through the end 64 and is slideably received in a slot 74 formed in linkage arm . 8 . slots 24 and 74 may be constituted by a single slot . in this example , the movable support 7 is shown supporting a vdu 75 . [ 0080 ] fig1 and 15 illustrate an embodiment of the invention where the disposition of the various linkage arms relative to one another serve to provide torsional rigidity to the linkage mechanism . the linkage mechanism 5 in this embodiment comprises a first bowed linkage arm 8 pivotally attached at one end 9 to a bracket 10 fixed to the member 6 and pivotally attached at a second end 11 to the movable support 7 . a second linkage arm 12 is pivotally attached at one end 13 to the bracket 10 at a connection point 14 which is spaced from and above the connection point 15 of the first linkage arm 8 and at its other end 16 to a connector pin 17 . a third linkage arm 18 is pivotally attached at one end 19 to the movable support 7 at a connection point 20 spaced from and above the connection point 21 of the first linkage arm 8 and at its other end 22 to the connector pin 17 . the connector pin 17 links the respective ends 16 and 22 of the second and third linkage arms which are located on opposite sides of the linkage arm 8 to a point 23 intermediate the ends of the first linkage arm 8 . connector pin 17 passes through the end 16 , slot 24 and then end 22 . linkage arm 8 is effectively sandwiched between arms 12 and 18 . the sandwich arrangement of the various arms provides for uniform distribution of the load and resists torsional and compressive forces applied to the linkage mechanism . it will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described .