You can't delete the default account, the one at the top of the folder pane, so select another account in Account Settings, such as the gmail account, Account Actions, Set as Default, then select the ISP account, Account Actions, Remove.

The new Gmail account is at the top of the folder pane and I believe I have it set as the default email account. It is the MHcable account I am trying to delete.

Is the gmail account at the top of the left pane in Account Settings? If 'Set as Default' is greyed out when you select an account, then Account Actions, then it is the default. If you can't remove a non-default account, there is something wrong with your setup.

The order of accounts in the folder pane may be different than in Account Settings if you've reordered accounts with the Manually sort folders add-on.

Here is a screen shot of the account settings page. I am not that well versed in Thunderbird but I think it is correct.

I appreciate your help, thank you!

Somehow, Local Folders got renamed to MHcable. While it's possible to remove LF, it's not advisable, and I recommend you change the name back to the original. If you were to create a new profile, TB would show Local Folders by default. As the name suggests, LF is a storage location that exists only on the local computer, and isn't defined for a particular account.

See more details here: http://kb.mozillazine.org/Local_Folders

I'm not sure what you mean by change the name back to the original?

Right-click MHcable in the folder pane, Settings.

http://kb.mozillazine.org/Local_Folders#Renaming_Local_Folders

Managed to restore the folders so thanks to all, it was very much appreciated.