So...
I deleted the project folders I created and then saved from scratch for the umpteenth time in the hope it might just work this time. I can now watch the exported video. It may not happen on the first, second, third or fourth etc time of opening the file, when it will automatically close without playing anything, but eventually it does play it and when it does it looks okay.
I'm not sure why it doesn't play on the first time of opening the file, but as long as it does open eventually I will accept it.
btw, before my initial correspondence I had tried saving as an AVI export, but that didn't work either.
Given the problems I've been having trying to do even the simplest things you've asked, and even though this isn't perfect I think I shall accept it as effectively working. Still it is very odd that the file doesn't open at the first time of trying??? Hey ho!
Thank you for trying to help me.