Arrggghhhh.... wasted too many evenings on this. Turns out it *does* still crash with PBO on. Tried manual RAM timings, no change. Re-seated and re-pasted again (not in that order, obviously), ran for a while with XMP disabled and IF at 1333 - still crashed (but less). Re-enabled XMP, ran IF at 1800 (ram at 3600) but with Global C-States disabled and *thought* it was stable, but no. 2 crashes in quick(ish) succession running Dirt Rally.
And probably a hundred other things I've tried this week. Put the 3700x back in and all is peachy. Except for this £400 paper weight. I give up. Hopefully Overclockers will accept all this endless faff-and-fail is sufficient evidence that the chip is clearly a basket case and replace it