Who is your provider right now?
Edit: Seen in the OP's profile he is with Plusnet.
My advice would be to speak to Chris Parr or other members of staff on here/Plusnet forums. They should be able to raise this as a data integrity issue with Openreach's database.
The bridges with PlusNet are well and truly burnt at this point; they just spent 2 years alternating between implying that I was an idiot, and implying that I was a liar.
It eventually culminated in a senior member of their faults team calling me, 5 days before Christmas last year, and telling me in no uncertain terms that PlusNet will not be fixing the issue, and that they considered the matter closed, regardless of how I felt about it.
The chap then asked why I didn't just move house if this was such a big problem; eventually I decided that explaining to them, yet again, that I'm < 150m away from a FTTC cab, and should - by rights - be getting 80/20, wasn't going to do anything.