fig2 shows a separator which comprises a gully chamber 2 having a cylindrical surrounding wall 4 and top and bottom end walls ( not shown for clarity ). an inlet duct 6 and an outlet duct 8 open into the surrounding wall 4 . an inlet may also be provided in the top wall in a region which is close to the surrounding wall 4 and at a circumferential position which is near to the inlet duct 6 . such an inlet may be positioned in a roadway passing over the separator and covered by a grating . a separator module 10 according to an embodiment of the invention is installed in the gully chamber 2 . as shown in fig3 and 4 , the separator module 10 comprises a partition arrangement having a partition plate 12 and a separate ledger component 14 . the partition plate 12 divides the chamber 2 into an upper region above the partition plate 12 and a lower region below the partition plate 12 . the partition plate 12 comprises an inlet chute 16 and an outlet chute 18 . the inlet chute 16 comprises an opening 20 which provides an inlet to the lower region from the upper region . similarly , the outlet chute 18 comprises an opening ( not shown ) which provides an outlet from the lower region to the upper region . the inlet chute 16 has an arcuate shape which is contoured to conform to the curvature of the inner surface of the surrounding wall 4 . the inlet opening 20 is situated close to the internal surface of the surrounding wall 4 , and is oriented to direct incoming flow tangentially of the chamber 2 . that is , it lies in a plane extending radially with respect to the central axis of the chamber 2 . likewise , the outlet chute 18 has an arcuate shape which is contoured to conform to the curvature of the inner surface of the surrounding wall 4 . the outlet opening is situated close to the internal surface of the surrounding wall 4 . the outlet opening is oriented in the same direction as the inlet opening 20 with respect to the circumferential direction around the central axis of the chamber 2 . the inlet chute 16 and outlet chute 18 are positioned in the partition plate 12 so that they are diametrically opposed from one another , with a central standpipe portion 24 disposed therebetween . the central standpipe portion 24 provides access to the lower region of the chamber 2 to allow the chamber 2 to be cleaned out using a standard vacuum tanker , for example . the central standpipe portion 24 also allows air to escape from the lower region as the latter fills with liquid . the central standpipe portion 24 may be cut to suit the headroom available within the chamber 2 . the central standpipe portion 24 may be closed at its upper end , for example , by a removable cover . the partition plate 12 comprises an annular bypass channel 26 defined between the central standpipe portion 24 and the inner surface of the surrounding wall 4 . the annular bypass channel 26 fluidically couples the inlet chute 16 and the outlet chute 18 in the upper region . the inlet chute 16 and the outlet chute 18 are arranged so that the exit from the outlet chute 18 in the upper region of the chamber is located above the entrance to the inlet chute 16 . accordingly , the annular bypass channel 26 is angled and slopes upwards from the inlet chute 16 to the outlet chute 18 . the annular bypass channel 26 is inclined at an angle of between 30 and 40 degrees from horizontal . the annular bypass channel 26 projects upwards either side of the outlet chute 18 to form a pair of weirs 28 which extend radially from the central standpipe portion 24 . as shown in fig2 , positioning the weirs 28 either side of the outlet chute 18 allows the inlet duct 6 to be disposed at any position about the remainder of the surrounding wall 4 . indeed , this arrangement allows a plurality of inlet ducts and / or larger inlet ducts to be used . the central standpipe portion 24 is provided with a bypass port 30 which opens into lower region . the top edge of the bypass port is positioned at or below the top edge of the weirs 28 . the bypass port 30 may allow air to escape from the lower region where the upper end of the central standpipe portion 24 is closed and sealed . referring now to fig5 , the ledger component 14 comprises an annular ring which is configured to be affixed to the inner surface of the surrounding wall 4 . the ledger component 14 is designed to support the partition plate 12 within the chamber 2 such that the partition plate 12 does not directly contact the inner surface of the surrounding wall 4 . accordingly , the ledger component 14 is shaped to conform to the profile of the underside of the partition plate 12 . notably , the ledger component 14 comprises an inlet recess 32 which corresponds to the inlet chute 16 and an outlet recess 34 which corresponds to the outlet chute 18 . between the inlet recess 32 and the outlet recess 34 , the ledger component 14 is inclined at the angle of the annular bypass channel 26 . the ledger component 14 further comprises a pair of projections 35 which correspond to the weirs 28 . the ledger component 14 has a gap 36 positioned midway across the outlet recess 34 . together with the flexibility of the material used to form the ledger component 14 , this discontinuity allows the ledger component 14 to contract so as to reduce its external diameter . the complementary features of the partition plate 12 and the ledger component 14 interlock the two components so as to prevent rotation of the partition plate 12 relative to the ledger component 14 . there is , however , sufficient laxity between the complementary features so as to permit the ledger component 14 to contract by closing the gap 36 . to install the separator module 10 , the ledger component 14 is inserted into the chamber 2 . depending on the diameter of the surrounding wall 4 , the ledger component 14 may need to be deformed from its rest position to correspond to the inner diameter of the surrounding wall 4 by closing the gap 36 . the ledger component 14 is then affixed to the surrounding wall 4 before locating the partition plate 12 thereon . the gap may also allow the ledger component 14 to expand so as to increase its external diameter . in use , under low or moderate flow , incoming water flows into the chamber 2 from the inlet duct 6 and enters the inlet chute 16 either directly or having been directed to the inlet chute 16 by the angled annular bypass channel 26 . the water passes through the inlet chute 16 and enters the lower region via the inlet opening 20 . the flow enters the lower region in a tangential direction and so circulates around the lower region . the flow from the inlet opening 20 will also induce a circulating flow in the water accumulated in the lower region of the chamber 2 . this relatively low - energy circulating flow will assist in causing any solids within the incoming flow to accumulate and fall to the bottom of the chamber 2 or rise to the fluid surface depending upon their density . furthermore , by sweeping past the outlet opening of the outlet chute 18 , solids will be less likely to enter the outlet chute 18 . the outlet chute 18 , and consequently the outlet duct 8 , will thus receive substantially clean water . if the incoming flow rate from the inlet duct 6 increases to a rate above that which can pass through the inlet opening 20 , the level in the upper region will rise . eventually , it will reach the level of the upper edge of the weirs 28 , and will overflow into the outlet chute 18 and the outlet duct 8 . much of the solids material entering the upper region through the inlet duct 6 will pass through the inlet chute 16 and emerge from the inlet opening 20 , and so , as before , will tend to fall to the bottom of the chamber 2 or rise to the fluid surface . nevertheless , under heavy flow conditions , some solid material will pass , with the water , over the weirs 28 into the outlet chute 18 , and thence to the outlet duct 8 . however , since the main flow within the chamber 2 does not receive the surplus flow passing over the weirs 28 , there is less likelihood that the high flow throughput will stir up solids from the bottom of the chamber 2 or from the fluid surface and cause them to enter the outlet chute 18 . moreover , since the top edge of the bypass port 30 in the central standpipe portion 24 is positioned at or below the top edge of the weirs 28 , any accumulated floatable material in the upper region is able to pass into the lower region before the water level reaches the upper edge of the weirs 28 and so does not pass over the weirs 28 to the outlet duct 8 . the inclined annular bypass channel 26 prevents material from accumulating on the surface of the partition plate 12 . initial tests have shown that an inclined annular bypass channel provides approximately a 80 to 90 % reduction in accumulated solids on the floor of the bypass channel 26 , compared to the equivalent level channel . fig6 and 7 show a separator module 110 according to another embodiment of the invention . although not shown , the separator module 110 may be installed in the chamber 2 described in relation to the previous embodiment . the separator module 110 comprises a partition arrangement having a partition plate 112 . the partition plate 112 divides the chamber 2 into an upper region above the partition plate 112 and a lower region below the partition plate 112 . the partition arrangement further comprises an inlet chute 116 and an outlet chute 118 which are formed as separate components from the partition plate 112 . the inlet chute 116 comprises an opening 120 which provides an inlet to the lower region from the upper region . similarly , the outlet chute 118 comprises an opening ( not shown ) which provides an outlet from the lower region to the upper region . the inlet chute 116 has an arcuate shape which is contoured to conform to the curvature of the inner surface of the surrounding wall 4 . the inlet opening 120 is situated close to the internal surface of the surrounding wall 4 , and is oriented to direct incoming flow tangentially of the chamber 2 . that is , it lies in a plane extending radially with respect to the central axis x of the chamber 2 . likewise , the outlet chute 118 has an arcuate shape which is contoured to conform to the curvature of the inner surface of the surrounding wall 4 . the outlet opening is situated close to the internal surface of the surrounding wall 4 . the outlet opening is oriented in the same direction as the inlet opening 120 with respect to the circumferential direction around the central axis x of the chamber 2 . the inlet and outlet chutes components each further comprise a backing plate 138 and first and second circumferential flanges 140 a , 140 b , which will be described in more detail below . the partition plate 112 comprises an inlet recess 142 and an outlet recess ( not shown ) which are diametrically opposed from one another with a central standpipe portion 124 disposed therebetween . the inlet and outlet recesses are configured to receive the inlet and outlet chute components , respectively . a keyhole slot 146 opens at the centre of each of the inlet and outlet recesses and extends partway up the central standpipe portion 124 . as per the inlet and outlet recesses , the keyhole slots 146 are diametrically opposed from one another and so divide the partition plate 112 into two halves which are joined to one another by the central standpipe portion 124 above the keyhole slots 146 . as shown in fig7 , the inlet and outlet chute components are received by the inlet and outlet recesses of the partition plate 112 so that the inlet and outlet chutes 116 , 118 are positioned centrally in the inlet and outlet recesses . the inlet and outlet chute components are received from below the partition plate 112 , with the backing plates 138 being inserted into the interior of the central standpipe portion 124 so that they cover the keyhole slots 146 formed in the central standpipe portion 124 . the circumferential flanges 140 a , 140 b abut with and seal against the underside of the partition plate 112 . the partition plate 112 comprises an annular bypass channel 126 defined between the central standpipe portion 124 and the inner surface of the surrounding wall 4 . the annular bypass channel 126 fluidically couples the inlet chute 116 and the outlet chute 118 in the upper region . the annular bypass channel 126 projects upwards either side of the outlet chute 118 to form a pair of weirs 128 which extend radially from the central standpipe portion 124 . as described previously with respect to the separator module 10 , positioning the weirs 128 either side of the outlet chute 118 allows the inlet duct 6 to be disposed at any position about the remainder of the surrounding wall 4 . indeed , this arrangement allows a plurality of inlet ducts and / or larger inlet ducts to be used . although not shown , the annular bypass channel 126 may be inclined as per the annular bypass channel 26 of the previous embodiment . the partition plate 112 may comprise a number of tabs 144 which can be used to affix the separator module 110 to the surrounding wall 4 of the chamber 2 . if the ( at rest ) external diameter of the partition plate 112 is smaller than the internal diameter of the surrounding wall 4 , the keyhole slots 146 allow the two halves of the partition plate 112 to be splayed apart in order to increase the external diameter of the partition plate 112 to conform to the diameter of the surrounding wall 4 . the connection of the tabs 144 to the surrounding wall 4 may maintain the two halves of the partition plate in this expanded position . alternatively , the introduction or connection of the inlet and outlet chute components in or to the inlet and outlet recesses may maintain the partition plate 112 in the expanded position . the backing plates 138 are of sufficient size so that they cover the keyhole slots 146 even when the partition plate 112 is in the expanded position . to a lesser extent , the keyhole slots 146 may also allow the two halves of the partition plate 112 to be compressed together in order to decrease the external diameter of the partition plate 112 . as described above , both the separator module 10 and the separator module 110 have mechanisms by which the external diameter of the module can be adjusted so as to conform to chambers having different diameters . this is particularly important where such modules are supplied to both the uk and us markets , with chambers being typically sized based on metric and imperial measurements respectively . the arrangements also provide an improved seal between the precast chamber and the components of the separator module .