Well I've fixed it! With many thanks to Asyn. Thanks for your suggestion too Bob.
It seems it was a combination of Avast's over-enthusiastic protection and Windows sharing settings. The problem was that you change something in one, it doesn't work so you change something elsewhere, change something else .... and so it goes on.
It seems the geek suggestions in the link given by Asyn was one of the problems, then Windows didn't want to share even though the Windows "help" ( I use the term advisedly) instructions were followed to the letter. So thanks guys for your assistance.
I've still got the ransomware shield disabled, but will re-enable that and hope it all still works.
David