I seem to have got it fixed, although i cannot be sure that it wasn't something that the ISP had done (they had just changed the mailserver details, then it all went down just before I got it working).
queyr status=2 seems to be a common result from fetchmail, covering a multitude of sins, the manual says "treat as irrecoverable".
The change I made just before it started working was to change the primary DNS name of the server from "foo.com" to "local.foo.com", where the ISP mailserver was known as "mail.foo.com" (but i had got the name marked as "remote", and a ping found the external one ok).
If anyone has any comments on this I'd be glad to hear them...
cheers
Brian