newcastle, have you checked to see if the cocking piece thread is loose on the firing pin? I don't mean unwound; but maybe a worn thread so there is unwanted movement. That will cause possible trigger issues.
I had this issue with my No1 MkIII*. So I followed Peter's advice and tinned the firing pin thread with solder. While it's still molten, use a soft wire brush to remove excess solder.
Refitting the cocking piece will be a good tight fit on the threads; removing any slop and improving the trigger's regularity.