I have had mine in bits again, and this time with the benefit of the parts diagram found elsewhere on this forum. I seemed to be missing a trigger sear spring (part 37) but have replaced this as per the diagram and the pistol still will not cock- more specifically the rearmost sear (part 43) will not engage to retain the piston. After a quick web and form trawl this seems to be a very common problem with this model.

Can anyone help?!

I am going to try something I learnt with old seagull outboards in order to 'renovate' the existing leather breech seal which is very worn: carefully remove it, soak in boiling water, then soak for a week in light oil, then replace. Lets hope it works!