Many thanks to all who advised on the original question, and who may like to know the outcome.

There seemed to be a general agreement that the 124 is pretty good in standard trim, so I decided to try that, at least to start with. Chambers "original" spring (had to polish grinding marks off the ends) and seal fitted, lightly greased and reassembled, and it wouldn't cock! I hadn't touched the trigger, so it didn't take too long to work out that the new spring must be the problem, and that 41 coils of 3.mm dia wire plus two 2mm delrin washers (not original, but in the gun when i got it, so I left them there) don't fit into the 120mm of available space for the compressed spring. Two coils off the spring, end flattened and polished, and one washer removed, fitted a piston liner while I was at it, as the spring really was rattling around inside the piston, back together and big smile time. 10.9 ft/lbs and the firing cycle is absolutely transformed, completely civilised with no trace of the crash and twang which compelled me to dismantle my new purchase in the first place.

Am I going to have fun when I take it out to play tomorrow!