are you sure its the suspension? it could be the exhaust knocking at the back... (£1.50 for new rubber hanger)
Other than that it could be the peice of rubber that the rear coils sit on, mines dead and cannot locate another kwick fit broke mine when they did new suspension and couldnt replace that part because they couldn't source it
Failing that everything Ziggy said, you can "test" the shocks by bouncing the car, if it dosen't stop bouncing they they have gone - but if the oil comes out this dosen't mean they good