This involved adding a new OnUpdatePrimSingleRotationPosition event to IClientAPI so that we can get the changed position from the client. Btw adding new events to IClientAPI is really tedious where you have to copy the change across to at least 5 or 6 other files. [Note this doesn't fix the bug where any rotation changes to the root prim (but not the whole linkset) cause rotation errors on the child prims.]