It worked Kofa, but raises a question.
I tried darktable --disable-opencl at the command prompt and had 3 or more ‘boxes’ flash on screen and go and the darktable icon appeared on the taskbar. I clicked it and it worked !!
So it is to do with Opencl, now what ? I will search a bit but I can at least set up a desktop shortcut with the above command in it.
I will look into istalling Opencl again, once I have figured out what it is.