Thanks gp for your suggestions

I have a Linux machine I don't use much, running an old version of Mint (18.3 'Sylvia' )

This also has thunderbird, (I have just switched it on and tested it) which works OK proving my service provider LCN do not have a proble at their end.

As my wife's desktop and my laptop both developed the problem about the same time it looks like the problem may be due to a windows patch which was automatically installed.

Problem is how do I resolve the issue?

Thunderbird originaly working OK on Linux Mint now getting same error.

Different operating system, different hardware, old version of Thunderbird - it has to be the service provider despite the fact that they say no ??????????

I'd say that yes it rules out a Thunderbird update or an operating system update, so either your mail provider or your Internet box. That's why testing with a computer in another place could be interesting (or using another internet connection - maybe you could use a phone 4G tethered connection if you don't have a cooperative friend - usually laptop have wifi connectivity and Android phone have this capability of sharing their connection through Wifi).

Think I may have just corrected the problem:

We have a TalkTalk router which does not cover the whole house so I bought a Netgear range extender for a wire link to the TV and to extend the coverage.The coverage still isnt quite sufficient as my wife likes to have her tablet in the bedroom for BBC playback and the walls are thick, so our son gave us a tp-link range extender. I just unplugged it and everything seems to work OK now, also the sending speed of the emails seems to have improved quite a bit.

Many thanks for your help and suggestions