There seems to a little progress, I can send and receive emails via Thunderbird client.
I can send emails via eM Client but not receive any even though I can receive them on Thunderbird and Web based BTYahoo.
It is now five days since I received any emails via eM Client.
Surely if one client is working as expected, but not another, then this has to point at a local issue.
I would tend to agree and am working on finding the problem. But it is only today after a period of 5 days that the emails have started to arrive again on Thunderbird.
I took Matt-S's advice. I changed my password and did not attempt to connect for 3 days. Changed password on client and it worked first time. This was the longest outage I have suffered.
Success after 5 days.
As I write this my eM Client has kicked into action and is synchronising and downloading my emails.
I had to do some sorting out to get to this stage.
Normally I use a desk top PC and that is where my eM Client resides.
I downloaded eM Client to my laptop an hour or so ago and without much intervention from me it set up the correct ports and downloaded emails from BT.
I checked all the eM Client settings on both my laptop and PC and they were identical which still didn't help to resolve the problem.
The only option I could think of was to reinstall eM Client from scratch on my PC after which the emails started downloading.
Synchronisation of the emails is in progress and as I have many thousands of emails it may take a little while to complete.
Thanks to everyone who offered advice including DaveM the moderator who also took time out to telephone me today regarding this issue and my other unrelated problem regarding downloading bills.