Actually I've just added a 3mm aluminium plate (scrap from clients who are aluminium fabricators) instead of the 1.25 mm glass I was using.

The glass was bending, the aluminium does not :-)

Much much better - I've now got perfectly flat printbed.
Guess I could have bought expensive thicker flat glass. But I'm happier with something that won't break if dropped :-) (already killed 2 glass beds)