Guys
Thanks for the suggestions so far. Spent the afternoon under the motor and removed the rear suspension arms one by one! The only thing I can see is the two bolts holding the top of the a frame to the chassis are indeed too short on the shank of the bolt, meaning one end is bearing on the thread. Tightened these up big time, and the clunk is heard less often, but is a lot louder when it happens, so looks as if we are on the right track. Will have a hunt around for different bolts to try in place.