Can anyone point me in the direction of where to buy an item I have seen loads of times, but don't know its name 
Its a pair of interlocking wedge-shaped blocks that are fitted centrally on a side hung pvc sash to pull the centre section in towards the frame. My client has low end windows that are bowing slightly in the middle - the hinges are working well and pulling the top/bottom in but there is still a bow in the sash. I have de-glazed and reglazed with sash propped true, but it still goes back to its old position (externally beaded with wedge gasket) so as a last resort i'm trying to source these items. Crank, I seem to recall a post of yours some time (years???) ago on this topic. Any advice appreciated.
Its a pair of interlocking wedge-shaped blocks that are fitted centrally on a side hung pvc sash to pull the centre section in towards the frame. My client has low end windows that are bowing slightly in the middle - the hinges are working well and pulling the top/bottom in but there is still a bow in the sash. I have de-glazed and reglazed with sash propped true, but it still goes back to its old position (externally beaded with wedge gasket) so as a last resort i'm trying to source these items. Crank, I seem to recall a post of yours some time (years???) ago on this topic. Any advice appreciated.