My buttpad is a straight plate to accomodate prone/kneel/stand. I'll check that I'm putting it in the same place. Maybe that's my problem and I need to go back to using a curved pad to ensure the same fit in the shoulder every time?
I've measured the gap from th erear of the trigger block to the flat at the rear of the stock. It's 18 thou. So I could easily fill that with some alloy sheet material and glue in as per Paul's advice. I've just had the action out of the stock and refitted it so I'll shoot it today and see if that 18 thou changes ... to see if the action does move backwards ... and notice any change in poi.