Mike,

I've dragged a couple fo No 1 Mk III*s out of the safe after oooh.. aaahhh... eerm... 30 years?

The mag springs are about as good as half my undies elastics in summer. You've worked out the COL matter, thanks to some fine guidance above, but I do think you've supplied about 60-70% of the solution by dealing with the spring. A strong spring does wonders for rim over in the No 1s, as evidenced by experiment and changes. Found same with No 4 rifles. New spring, no rim lock.

Well done!