Well, more POI shift appeared. 40mm up at 55 yds. Having shot the thing probably every day for 4 days now we were starting to see a pattern. Essentially she just wandered up and down, settling for a bit, perhaps 20, perhaps more, before wandering the other way. Environmental influences seemed coincidental, not consistent enough to call it.
Anyway we unbolted the front end to have a look at any potential tension. Phil's inserts when we put then in lined up exactly. When we took them out last night the holes in the n bracket up front moved 1+mm up, like the top pic. Shooting it even with just the one bolt in the back on the pillar saw the POI go up and down still, but each time it did we could see the holes go down and up. We're talking 2.5 MOA at 50 yds. Final test was lifting her at the muzzle and letting her rest in the stock and she was spot on... pressing her hard down into the stock, she went 40+mm high.
So we (or Phil) re inletted the stock. Any area that vaguely looked like it had ever touched got sanded away. She's now sitting on just the pillar, she's got room to move, and she held zero for 40 or so shots last night before we lost the light. With the front bolts out now I can at least see how she's sitting. But starting to get some confidence back.