Right, well, after a few fruitless hours being passed from pillar to post, they suggested I un-and-re install. This took a couple more hours. And didn't work. So they sent me a new copy of CS6. Only they didn't. It was a trial version of CC. The second copy was downloaded by just 9.30 (11 hours after I first contacted Adobe about the problem). It worked, and now keeps nudging me to update to RAW 8.5. I think that on reflection I probably won't.
No idea what caused this, but thanks for all the suggestions.