Funny you should say that - after doing some research on the internet, it seems that's the only possible reason. However I didn't think that being on all day - being situated on a dining room table in a well aired room would cause that problem.
It hasn't recurred since but at least having an awareness of that sort of thing recurring will make me remember to switch the laptop off when not in use! Kind of a good housekeeping kind of approach that I will need to stick to
-------------------- Aren't computers wonderful... when they work!