Note: I removed the original post, detailing my battles with TalkTalk. Too long and boring. But this was the upshot:
So they finally sent a (BT) engineer out. Took him two minutes to remove a suppressor from the master socket, and now I'm finally online.
I can't believe it. Nearly three months of hassle for a job that I could have done with a pair of nail scissors. Anyway, I'm now waiting to see how TalkTalk are going to compensate me. I'm sort of hoping for a Mercedes SLK.