Well all I know I tried to use IMAP before and there were problems but too long ago to remember why. I was under the impression it wasn't supported. The instructions for setting up the account seem to be written around creating a new account rather than checking the existing one. But I'll give it a try anyway and report back on success or failure.
Ah an immediate update ! Looking at
http://www.setup-outlook.com/outlook2003yahoo.php I see a box has changed. it says that the user name must have the @yahoo.co.uk on the end but that wasn't the case in the past and other web help pages do not show/say it needs that. Up until now only the E-Mail Address box needed that ! But the test account settings has finally passed.
I think they must have changed something. It must have been optional until this week and now is demanded.
I think I may be back in business, thanks. I'll spend some time testing it out.