Sorry I wasn't clear, the sear doesn't fall into the half cock notch, rather it hits the lip under the half cock notch when falling. I posted multiple pics of the inside at half cock, not sure how the lighting was on each, trying to show the length of the fly relative to the half cock notch.
When pulling the cock from rest to half cock, the fly is not pushed up at all, there's no way the fly will block the sear from hitting the half cock notch/lip when firing.
I may well end up sending this in to Chambers to look at.