I'd be grateful if anyone could shed some light on the following.

I frequently shoot break-barrel springers with open sights, but have noticed over the last 2 to 3 months that I need to wind the rearsight almost all the way across to the right to be on target. This wasn't always the case, but I don't know what's changed. Relevant factors are;

It's not that windy when I shoot
I wear spectacles to correct severe short-sight, but haven't had a new pair or a prescription change
The laser boresight confirms the sight is offset to the bore when wound fully right
All 3 of my open-sighted guns are similarly affected
I put a bubble level on one gun to eliminate any canting (made no difference)
I don't appear to be leaning over the stock
Group sizes are OK at ranges from 10m to 40m
Offset doesn't change significantly with ammunition (0.177 AA Field, JSB exact, FTT)
Haven't cleaned the barrel
Both fore and rear sights are secure
Foresight blade is vertical when the gun is levelled, and on the barrel centreline
Rear sight top is horizontal when gun is levelled
No change in gun support (sitting, gun supported in hands, left elbow rested on a table)

Any thoughts as to what I could try would be much appreciated, as I'm beginning to run out of ideas!

Best Regards,
Bill.