Retested my lifting jaw vice and found that tapping the work firmly down on the parallels fixes the problem, < 0.01mm lift over 40mm. That's probably why I hadn't noticed the lift before – I habitually tap hard enough to grip the parallels under the work. But I now know that carelessly gripping work in the vice without tapping it down introduces a tilt error that might matter.
It's been useful to check with a DTI – they reveal problems and prove/disprove fixes rather well! New year's resolution: I'm going to double-check my set-ups more often.
Dave
Edited By SillyOldDuffer on 01/01/2018 17:52:22