I know nothing about Sky, but if it uses Yahoo, this info may prove helpful:

Yahoo requires a different password when using an email client and requires OAUTH2 authentication. This URL may assist: https://help.yahoo.com/kb/SLN15241.html

Thank you for your reply David. I think this is maybe where my problem is. However having gone into my Yahoo account, it looks like I will have to sign in from my laptop for several days to generate the Oauth2 authentication, which will have to wait until I get back from holiday. If I have any problems from there, I will send in another question.

The settings for Sky mail are here:

https://uk.help.yahoo.com/kb/sky/SLN4075.html

Set the authentication method to OAuth2, and enter the account (not app) password in the OAuth window when asked to grant TB access. An app password is not needed.

Thank you for your reply. I assume that set the authentication method is in TB's set up settings. Unfortunately when I go into TB it shows my email address, so TB must recognize that it was installed before and asks for my password, so I cannot get to any of the TB settings.

If the account is already in TB, you can (as long as you're not switching between IMAP and POP server type) adjust the settings in Account Settings, Server Settings, and Outgoing Server (SMTP). Remove entries for the account from Saved Passwords in Settings, then restart TB and enter the account password in the OAuth window popup.

Thank you for your reply. I cannot get into my TB account as the page I am taken to when going into TB asks for the password no matter what I try to click on in the page including Account settings.

Eliminate password requests by putting your computer in airplane mode or otherwise disconnecting from the internet. After making the changes, go back online.

Just wanted to thank you (sfhowes) for your replies. I did not get a chance to use your solutions as I tried my luck with David's solution above just before I went on holiday and even though it was only the 2nd day of signing into my Yahoo account, I was able to generate a password to get into TB, so did not have to try for several days.