So I decided to dig out my old 710 - and spent a day reading the forums. Updated it to the latest Navcore from TomTom Home (7.130) and installed the maps (890.4234) for that version - all working great.
Now I've started fiddling and found a NC 9.170 SE which so I've read will work on my 710. I've installed it and it indeed does seem to run.
My now question - according to the ...