In the example given it would be
fred.smith@ntlworld.com as the main account but the user name would be just the fred.smith part unlike hotmail where the user name is the full email address.
The problem you are having though is not that the user name and password are incorrect it is that the system, for one reason or another, is not confirming that they are correct which is not the same thing at all. (This assumes that the same details have worked previously and are therefore correct though)
That said what a few have found is that deleting the account in Outlook/Outlook Express and then re-adding it can help.
Others have found changing the pop and smtp address to the IP equivalent to bypass the DNS servers has also cured the problem.
However, if the problem is that a server carrying your details is currently not available for any reason, then none of the above will help until a server carrying your account details is back on line.
Hope that helps.