Update! On holiday in Turkey, and reattached the screen and it worked ok for 30 mins, then went again.
Next time I tried, it has stayed working for over a week now. I'm wondering if the 35' heat here has had an effect?
What is for certain is that I'm never going to detach the screen again, as long as it works like this that will do me.
Incidentally there were some Microsoft guys in my work a couple of weeks ago and one of them had a gen1 surface book and has had the same problem, and it was he who imparted he now never detaches the screen. He also had the same screen discolouration I have along the edges (glows slightly yellow).
Thanks again for your help sharpuser.