Update:
Called BT and they gave me loads of information regarding the tag placed on my line. It's been there since May 03 and the guy from BT said it could have been put on after an enquiry about broadband by me at the time. Don't remember the company - KB Media, now Legend Communications, but I was looking at various packages around then, but decided not to bother as I didn't use it at home enough.
Anyway, Legend are now part of Demon, who are something to do with Thus. I've spent the last 3 hours speaking to half of Asia, all of them telling me to ring someone else, before someone from Legend gave me a BT tag team number (different from above). They have given me an XMFP ID(?) to give to Legend to they can take the broadband off my line.
Sorry if it all sounds like a load of nonsense, but I don't have any technical knowledge of any of this, which is why I posted in the first place.
Still waiting for Legend to call me back!