TimS,
You will need to set the outgoing email server to the one of your ISP, ie
mail.btinternet.com, and leave you incoming email server the same for freeserve.
Turn off authentication, they will authenticate you as being on their subnet, not based on a username, is only bt internet people on a bt dial-up or broadband could use this outgoing email server.
Have a look at
http://www.btyahoo.com/faqs/manual_email_setup for pretty pictures and more details
In summary:
Outgoing email : use the bt internet server
Incoming : use the freeserve server
