Clonking found peace at last.
 

I have found where the clonking sound is coming from, Right under the gear lever area where I thought it came is the exhaust pipe and it just clears a cross member on the body work by about 3cm. The mid way box is held by 2 rubber bushes which had lost their strength and allowed the exhaust pipe to knock on the cross member..so on replacing the bushes I have a nice quiet drive..... sorted. However I was surprised to see that from that point to the next exhaust support right at the back is a long span. I would have thought the Rover design boys could have supported it with another bracket half way along....or is mine missing????