"there has been no change at all to my email account data and after entering the user name and password quite a few times (with the tick in box to remember it) eventually I get all my messages downloaded"

TN, are these BT or Tiscali/TalkTalk e-mail accounts by any chance?

"Yes they are Talk Talk accounts"

Ah, that probably explains it. There have been a lot of intermittent problems with the TalkTalk POP3 server over the past few months and the symptoms are that the user's e-mail client repeatedly prompts them for the username and password. If it's left for a while then the messages eventually come in as normal. Sadly, there's nothing you can do to fix it and you'll just have to wait until the server problem is resolved. If you're felling brave then you could get in touch with TalkTalk Support and see what they've got to say about the problem.

If you want to prove that it's not your Outlook that's playing up then configure a second e-mail client such as Outlook Express, or Windows Mail etc with one of the affected e-mail accounts. The next time you get that problem, try the other client, and if you get the same behaviour then it's almost certainly that problem with TalkTalk.

If you're ever desperate to get your e-mails then try TalkTalk's webmail as you may find that's unaffected.

My post http://www.pcadvisor.co.uk/forums/1/helproom/4110662/microsoft-outlook--repeatedly-asked-for-password/ is similar to yours and I use BT, I extended the time out and the problem seemed to considerably lessen, although I am not confident that the timeout was the solution. It still does appear but to a far lesser degree. I have not ticked my post as resolved as I am not sure that it is yet.