After a lot of faffing about I have resolved this.
I'm using a BTopenworld address, and I would recommend logging onto BT yahoo and checking the server settings for your account. I had mail.btinternet.com for both, but it appears that you need to change them to
pop.mail.yahoo.com (incoming)
smtp.mail.yahoo.com (outgoing)
Make sure you are using port 587 for the outgoing server and that the SMTP authentication box is checked.
Worked for me.