Dave,
Your post got my curiosity up so I took a look at the lock that I will be using on the next build as I will be using a simple trigger. I just got the lock last month from Liston. When cocked from the at rest position, the sear moved just a hair more than 1/2mm. Cocking to halfcock the sear did not change position. Cocking further to full cock the sear did not change position. Looks like it should work fine. I'm wondering if you may have one of the first run of locks?
I also looked at some other locks I have - Chambers, L&R, Roller. The sear position changed on each when going to halfcock then changed again when going to fullcock.