Well, your problem is because your ISP, BT Internet, were fobbing you off all the time, instead of realising you actually did have a line fault, and pushing it through to BT Wholesale.
If you were with an ISP with better tech support, they would have pushed it through to BT Wholesale a lot quicker, and this would probably have been sorted weeks ago!!
So I would still consider changing ISPs when your BT contract is up. Just to put your mind at rest, if you move ISPs, it won't affect anything to do with your phone line, or the DLM or anything like that!