That is an ingenious way of ensuring that the gun cannot fire unless the breech is fuloly closed Joe. Is that your invention or the standard semi-auto norm? The hammer part of your hammer block is acting the same way as the hammer part or what we call the 'hammer face' of the piston post in a standard gun. The piston post and your hammer cannot hit the striker with sufficient force unless the breech block is fully closed/locked. It gets better too because even if the breech block isn't fully closed in your method (or the standard piston post method0 and you decide to close it manually somehow, it still won't fire due to insufficient energy remaining within the hammer to initiate the primer.
I know it's a bit anorakish but keep thinking to myself that if we could get 6 or so UKand US/Can et al Bren forumers together in a classroom together with a few scrap Brens, paper pencils and a black-board we could solve this problem in an afternoon and all go for a beer in the evening. Are you game Tankie and Skippy........, between us we've fixed a zillion of them - and know them inside out!