An update on this.
It turns out that my Barley twist barrel that went bad aound 18 months ago has now died. It had been re-crowned by Barry Taylor the first time it went off, was good for over a year and now pants again. Put on a new 55cm barrel and the rifle was as sweet as a nut with some very good 55yd groups.
Might let Barry have another look at the Barley twist and see what he says. Pity really as the rifle was very smooth to shoot with the barley twist/stripper combo. The clean 55cm barrel (no stripper) is accurate but has a tiny bit of flip.