Can you select the IMAP account in Account Settings, then click Account Actions/Set as Default, then select the POP account, Account Actions, Remove Account?

Could not complete the second part of your suggested solution. Received pop-up message ' An account with this name already exists. Please enter a different account name'

You haven't done anything wrong. In fact you have to use the same address because both of these Thunderbird accounts are one and the same email account.

What I think it is really complaining about is that you have two accounts with the same name. If you were to set the names to the form of user@example.com pop and user@example.com imap then all would be good.

If it won't let you edit and change either of the names via Account Settings then you may have to do it via the Config Editor, or by direct editing of your prefs.js file.

Thank you very much for this piece of advice - IT WORKED. My sequence of actions within Account/Settings - 1. Checked which was the POP account then in the form changed the name to 'user@gmail.com'; clicked DONE; then checked that this had been accepted and everything was still operating as it should. 2. Selected the IMAP account, checked it was the correct one, and set this as the default account. Again clicked DONE and checked everything was working OK. 3. Selected the POP account, then deleted this account and clicked DONE and exited Account/Settings. 4. Checked email was operating as it should. 5. Exited email account and restarted computer. SUCCESS - and again, thank you for your help, it is very much appreciated.