BillTodd,
I just removed the saddle and examined the LHS dovetails and indeed the machining was not good. At the top of the dovetail was a small (0.1mm) of unmachined
ramp shaped metal 2/3 thirds the length of the channel. I scraped most of it away with a thin file and finished with off with 600 grit.
If I keep up this scraping process I will of course do two things - ruin the dovetail or get no near perfection!!!

I will reassemble the lot tomorrow and retry the previous laborious calibration setup again.