It seems the problem with this rifle has been solved and I am reporting back, as I promised. I took the rifle to Protek in Bognor Regis to inspect and they report as follows:

"A circlip - or actually an 'e clip' was missing from the sear axis pin. the pin had come out partially but was still held in by the stock. Repositioning the pin through both sides of the frame made the rifle shoot properly again. I have found an e-clip but it is a little large. I have squeezed it very slightly and the pin does not seem as if it will come out easily but really needs a smaller e-clip in the long run to be certain of its long term use. I have not got any smaller e clips. The e clip used on the trigger axis pin is larger by the way. "

That sounds simple enough and it seems that others have also had similar problems from the correspondence this has generated. This might be the solution for them too, or it could be the trigger axis pin if not the sear axis pin.