This guide is based on Thunderbird version 1.5 - steps for other versions may vary.
1. Open Thunderbird. Click Tools > Account Settings
2. Click 'Add Account'
3. If 'Email Account' is checked, click 'Next'
4. Enter your name and new email address. Click 'Next'.
5. Put 'mail.opgweb.co.uk' in the Incoming server box. If the option is visible, also enter it in the Outgoing server box.
Untick the "Use Global Inbox" option if you want this account to have its own separate inbox. Click 'Next'.
6. Enter your email address in the Incoming User Name box. Click 'Next'.
7. Put anything you like in the Account Name box. Click 'Next'.
8. Click 'Finish'.
9. Click 'OK'.
10. Click the 'Get Mail' button. It should ask for your password. Enter this and tick the "Use Password Manager to remember this password" box, then 'OK'.
Assuming this works with no errors, compose an email then send it to your new account. You may be asked for username and password again here. Again, tick the box so the password is remembered.