Well got it apart again this morning to check further, I'm not sure how Musky and RDfella spotted it but they're both right, the plate is bent. Not across the face, that's straight enough checked with a straight edge in a multitude of different planes, but the central boss section is out, and not by much either, but the .25 mm (or whatever) in the centre is multiplied at the outer edge, and doubled again from one side to the other ( raise one side, the other lowers by an equal amount so a big discrepancy.
Anyway, what I've done is, file a tiny bit off the high side of the centre of the back plate then bias filed a washer, and placed the washer filed side to the plate between the central bearing collar and the brake plate, all of this remains static and just gets clamped together when the brake plate centre securing nut is done up.
It took a bit of trial and file on/off faffing but the brake plate now sits pretty much evenly on the drum and no longer locks up when the nuts are tightened properly.
The third pic is the washer in place on the hub.
The last two pics are for comparison, after and before.
So again, thank you all for the help, advice and eagle-eyed detective work.