Well--Tom sent me the broken frame as well as the other bits. Deejayuu managed to work a miracle and repair the frame and sorted the catch.
I've had a go at putting the gun together and it has been "Fun"

. I've offered Tom the mechanism back if I cna sort it but he has declined the offer.
SO--on with the tinkering.
Much messing about and the gun has not been cocking and holding. The piston is worn on the flange that the sear holds it. The sear looked a little worn and so did the cocking link. As the trigger was not the original, and the sear would engage and hold the piston if the trigger was not installed, I thought the trigger might be the problem. By swapping bits over with a working Junior, it appears that it is the piston that is the problem.
I may be able to get the flange of the piston cleaned up a bit by someone with a lathe, but as this flange in thinner than on later pistons, I would not be entirely comfortable with the amount of "meat" left.