Outgoing mail server is mail.btinternet.com, and username is just the username, not the email address. I authorised another email address on my own clarkeology.com domain to send from my btbroadband in the end, but don't think I really needed to do that. At least if I reply to one of these messages, or Clare does, the mail will come back to me and not some unchecked yahoo / btinternet account.
Now all I want is for someone to burgle my flat!
UPDATE: and it's gone again, sent one mail and now it's back to erroring again. The mail did arrive, seemingly from my own domain so I know that bit worked. Once. What on earth did I do to get it working?