Thanks to the above few posts. It is not the indexing of the safety (I didn't mention that in my last post), The piston head/ buffer are ok and the long sear lines up with the little on ok. I have a spare long sear to try and see how that goes. The thing worked pretty well for years but I have always had my doubts about the housing. I remember asking about the whether you could do away with the safety when I joined the bbs. I reckon it is possible if you junk the shaft and then try to lock a screw in the hole, that has been cut short enough for the small sear to catch but not the front of the hammer.
Anyone got a spare housing knocking about?
A "Fingers crossed" update. I changed the big sear and the gun seems sorted. Putting the pin through the old and new sears didn't seem to show much difference at all, other than the upper surface of the old one had marks where the piston had rubbed. I may try to clean it up at some point but for now the new sear is fitted. I am wondering whether the pivot hole has gone a tiny bit out of round?
Anyhow after a half shandy or 2, the soda water bottle caps were flying last night!