I finally got this problem resolved by BT.
I firstly had to log in to BT Yahoo using the email address I was given when I first signed up to BT as I had to verify my hotmail email address.
I then had to go to the Apple Mail App. Select 'Mail' from the toolbar and then 'Preferences'.
Open the Outgoing Mail Server (SMTP) drop down list and select 'Edit SMTP Server List'.
Select the + sign to add a new server. The server name I was told to use is smtp.mail.yahoo.com. I believe the description can be anything but I've labelled this server as btinternet.com. Press okay and the window will close. Select this new server as the outgoing server.
After all that, my email now works. Hope this is of some help.