Pete, I had one that suffered from the same thing. The only way I could get the body off was to remove the buffer and beam moulding (obviously), then slide some thin brass shim (plasticard will probably do) between the body and the chassis along each side to hold the body clips off the chassis and then GENTLY lever the body off the chassis. If you look under each end of the model from you'll see a recess in the chassis between the body and the chassis just where the buffer goes through the body It's there I used a small thin bladed screwdriver to gently prise the body off.
Steve
Steve