I then went into My Account details and checked my existing e-mail addresses held by Talktalk and to my surprise the address giving the problem was not shown.
In that case there's probably nothing you or I can do to fix it. Hopefully the TalkTalk second-line team will be able to recover it. However, have you tried changing the password? You can click on the "Forgotten your password?" link on the webmail login page.
If that doesn't help then you may want to send an email to that problem account from a working email address and see if it gets bounced back to you with an "unknown sender" (or similar) message. Regardless of the outcome you won't be able to do anything about it but it may prove helpful in determining whether TalkTalk have really lost your email address or whether it's simply missing from your account page.