Well, I found the cause. After messing about with both a lubed and unlubed hammer rail I discovered that the power adjust grub screw, the one under the anti tamper shear bolt, was super loose.
It was literally moving with every shot. I fixed it with a hint of soft loctite on the thread. Now the rifles super consistent again! Yay!
Only problem is that Liz's rifle has been playing up, so 'my' mpr is now hers and I have another one to sort out, low power and lots of nasty anti tamper.