An update (I went to site today)

From the master socket (phone socket only) the cable went: ADSL filter - RJ11 - RJ11 Router

I replaced that with a cable that went direct from the router RJ11 to Phone socket

The wire at the back of the master socket is a drop cable which exits through the wall, goes up the side of the building and onto a wire support

I also tried the router directly into the test socket (since there was one unused internal extension)

Results now are

Downstream Sync 6652 Noise margin 8 Attenuation 31
Upstream Sync 259 Noise margin 6 Attenuation 41

A massive improvement, but still not enough!

Upstream attenuation is still high - but if you were direct into the socket there is very little you could do.

Next time, try plugging in and unplugging a few times to remove any corrosion that may have built up. Also, are you able to see which colour wires are connected to the face plate? And are they good connections - tightly screwed down?

The next hurdle will be persuading BT that there is a potential fault - somewhere on their network. Have proof you have tried 2 routers and with no phones or filters attached and then hope you get an agent who really understands.

In reply to a post by MHC:

Next time, try plugging in and unplugging a few times to remove any corrosion that may have built up.

I suggest that is done using just the cable, or if the router is on the other end of it then with that powered off.

Good point ...

Another update!

Have now examined the wiring. There are a lot of cables inside the external black drop cable connected to the socket - the orange and the white are connected

Master socket wiring

Then there is a white cable which runs to an extension

Nothing obvious - the dropwire is four pair with Or/Wh, R/Gy, Bl/Bn, Gn/Bk so you have a pair allocated. the extension looks as though it uses Bl & Bl/W on 2 & 5 with Or/Wh on 3 for the ringer. Technically those are wrong but it does not matter.

Did you carry out the tests with the small lower front face plate removed and into the socket on the remain back back of the socket - that removes the extension wiring completely.

There's a ring wire (isnt there).

Is the extension filtered? Is it used? If not disconnect it!

Yes I carried out the tests with the faceplate removed and into the test socket (hence not too worried about the extension wiring)

The extension will be used in the near future

Seeing the stats from the second router would help to confirm that is not an oddity of the Belkin in the displaying of the attenuation figure.

The upstream may stay slow if there is some bandwidth banding going on possibly.

Would this help?

I could configure my Billion 7800N and leave it running at the site for a couple of hours. The advantage would be that it works with Routerstats and gives a lot more info including Bits, tone and SNR graphs