Well, I started cutting chunks out of the coach and what a job it's been. The plastic is very thick so I got at it with my trusty Lidl multitool thing with a cutting wheel attachment. As the wheel cuts the friction also melts the plastic a bit; this leaves a heavy residue of stuff to clean away from the cut edges. It took me the best part of an hour just to prepare one side; I filed away all of the surface detail as well - door hinges and so on. While I'm at it I was wondering what I should do for glazing. I was thinking of glueing clear glazing material to the body before fitting the overlays. This would give a finish, while not exactly flush, that would be a distinct improvement on the old Lima glazing which, incidentally, is part of the same moulding as the roof and actually fits down behind the body sides.
In the meantime I was able to source a pair of Lima B4 bogies for a few shillings on E-Bay.
Further carnage to follow...