Thanks for the response. After many hours on forums and eventually on MS Support Chat it has been solved. It was a driver incompatibility with the October update (there have been others I think) which updating the obvious drivers did not cure. Eventually MS used a special app to update every driver in one go, and Touch Screen / inking are back in use.
Incidentally, when I had several people suggesting it was a hardware failure I used my ‘secret weapon’ to show it was not, by booting into Android — if Touch Screen works fine in Android on the same device hardware failure is surely very unlikely, is it not? I have done this a few times, including avoiding having to ‘scrap the machine or send it back to China (?)’ as it supposedly had an unrepairable Hard Drive failure. In fact I have experienced almost no problems with the Cube’s hardware.
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