Dial gauge has arrived, with a hefty magnetic base. Clamped a piece of steel to bed, locked magnet onto it so I have a good stable base to measure from.
Zeroed at machine centre, half way through Z travel.
A single step at 1mm jog gives 1mm up, 0.85-0.87mm down. If down was the last direction, 1mm down and 0.85-0.87 up. The same circa 0.15mm backlash is there.
Using 0.25mm steps, a single jog gives 0.25mm up, 0.1 down. If down was the last direction, 0.25mm down and 0.1 up. The same .15 error.
Using 0.025mm steps, a single jog gives 0.025mm up, nothing down until the steps add up to more than 0.15mm. The same true of the reverse directions.
I removed the stepper pulley (which was tight), checked it for wear/ovality of hole etc., nothing. Re-fitted it and repeated the above tests with the same outcome.
I looked for movement in the X or Z carriage, vertically or trapezoidal, nothing. Checked pulley rotation for each commanded step, and it does appear to be moving as-per the jog commands.
So the only remaining component that has any slack potential is the belt. Re-tensioned it, but no change.
I have ordered a replacement belt GT2/6mm/2mm/522mm and whilst at it got a steel core equivalent (which is open, not a loop, so will have to think how to solidly joint it).
Given how fantastic this machine is, it is shame to still be wrestling with this backlash issue…