Have just done a good couple of hours plugging routers, USBs, filters, etc in all kinds of holes in all possible permutations. The conclusion is this; there is a problem with my phone master socket somewhere between the test socket and the socket on the faceplate.
I get my full 1Mbps speed test when connected via a USB or router, with or without filter into the test socket, but anything plugged into the faceplate socket drops to ~850kbps if it connects at all, and out of the 8 speed tests, four timed out and the remainder got between
400-600kbps.
I understand that the bit that BTs cable connects to is theirs, but can I replace part of this master socket? If BT do it, will they charge (and roughly how much)? I'm with a LLU ISP, but still with BT for line rental & calls. Do I go through my ISP or BT?