Thanks dc42 for all your help with this, it's really very much appreciated.
Prompted by your request for the files I went through the config.g again in "forensic mode", partly to tidy it up so you could see the wood for the trees and I came across a line
M563 P0 X19 D0 H1 ; Define tool 0. CS added X19 to account for temp fix to use 2nd extruder mounting position
which I now realise is totally wrong and was telling the fw to map the X movements to an axis numbered 19. I now know that the correct way to do what I was trying to do, offset the X carriage, is G10 P0 X19 Y0 Z0. RTFM!
The background to this is that I recently set about servicing my Huxley, print quality had deteriorated and it turned out to be that the X carriage plate was disintegrating on one side. RRP Huxley X carriages have two mounting positions for the hot end, the second meant for a 2nd extruder, so as a temporary work around I switched to using that but it is 19mm further left. I misread the wiki page and tried to offset the carriage using M563 when I should have used G10.
Of course I'm now kicking myself for breaking the basic rule of only change one thing at a time. 30 odd years doing this sort of thing and I still fell for that one!
Once again, thanks so much for your help, I've learned a lot in the process and I'm looking forward to using the new mesh bed leveling to fix my long standing issues with a slightly warped Huxley bed...