Do you have a filter connected or a VDSL faceplate? You say you have no phone connected. Do you know if there is any extension wires connected if you open up the socket? Can you take Pics of how its connected to the BT master socket and I'll be able to advise further
Its just a master socket with 2 connections and I plug the router into the top one. I've also taken the face off and connected into the test port and used the supplied filter which made no difference.
Ill take some photos when I get home, its a rental so I don't know the wiring or history unfortunately