Okay- some interesting results with dial gauge.

In middle of x axis (must be worst deflection zone), I can deflect the machine up or down a staggering 1.3mm! (worried).

I then thought- to prove the theory- deflection must be better or even minimal at either end of X-axis...

Its better, but still 1mm. So looks like its actually entire gantry flexing. Opens new possibilities.

Looking at Y-axis bearings, I previously closed the allen key sdjuster until it stopped, no further. I gave all 4 side screws a wee tweak tighter and deflection has been reduced to 0.4mm at ends and 1.1mm middle of Y axis.

I think theres another adjuster on top of bearing. I'll need to dismantle bed to get to it.

SIGH!