Thanks for the confirmation!
Ah... Now that's very interesting!(The first part as I'm all good with the second part ). I thought this gun was too basic to have any trigger adjustment ability, there is a tiny hole in the trigger just above the blade section but no screw (unless it's behind the tiny hole that seems unlikely). The spring has plenty of compression and expansion left in it ie. it's not bound or knackered (could probably use a new one after all these years but more for maintenance). That adjuster screw may well be missing and causing all the problems which would save me a lot of work and stress (I build modern guns and am not comfortable working on antiques to be honest!). Do you happen to have any links to what the adjustment system is supposed to look like? I'm guessing a screw through the threaded (hopefully, got a large collection of taps and dies though) hole and adjusted correctly will do the trick unless the sear is now knackered due to incorrect adjustment causing wear by skipping over the locking point repeatedly.
Cheers! Weird though...
I agree with the adjustment screw, very possibly the problem as I don't have one! . The pinch bolt's threads are knackered, just something else that needs to be fixed! So no interference there!