You were fortunate to solve the problem, I think.
What is a PH16m rear sight? Are you fitting a peep sight where the telescopic sight would otherwise be fitted, ie on the ramp? I did this with a Webley Omega (the peep sight also obtained from Protek) - which has a rather more solid ramp - to great effect. The Mk3 has quite a kick, relatively, so you might get movement on the sight.
A