Ok, I went to the pub and had a couple of beers, came back and it sort of switched on. It was very jittery and the login box jumped all over the place however after about 10 minutes I was able to enter my PIN. I left it for a few minutes, switched off, left it again and then it switched back on again.
This morning it's switched back on totally normally and says it is downloading updates therefore I'm just going to leave it alone and see what happens ... what an awful user experience plus I'd bought this thing for travelling with, now I don't know whether I can trust it!
I'll probably have a look round for another small machine and put Win 7 or Mint on it.
It's completed an update, I'll run Malwarebytes however I've a feeling it's the very poor update installation since I've just spoken with a friend and he's just experienced precisely the same, interestingly on a very expensive Lenovo laptop.