Thanks to one and all for your encouragement. Might get a clear night or two this week to keep hammering away at this one.
As an aside, I'm now using a short backfocus version of the QHY600m to get within a millimetre or so of the AFR's optimum focal plane. But...careful what you wish for.....as the OAG is no longer par focal with this model camera without a 10mm spacer! (defeating the reason for shifting to that model)
The fix I subsequently came up with was to place a concave lens ahead of the STi autoguider...which thankfully pushed the OAG focal plane back just enough to make it parfocal with the camera.