I'm guessing the upstairs socket is back wired off the downstairs socket on the green/white pair that is connected to the front plate downstairs. Presumably with the front plate off downstairs, the upstairs socket is dead?

Sounds like 2 separate lines on the 2 pairs that usually supply premises.

Would you mind dialing 1470 17070 from each extension and listen to the circuit definitions?

You are correct! Took faceplate off downstairs and line upstairs is dead

same number on each line...

Good! The 2nd pair to the old upstairs office must be deaded somewhere in the house.

So the line split outside house does not come into play and all your current line wiring must come off the downstairs NTE5 master.

The upstairs socket should not be an NTE5 master and may be affecting things by the presence of its duplicate master capacitor in the circuit.

Would it be worth swapping it with a simpler terminal?

Just tested stats on both lines again - Downstream synch speed is about 1Mbps slower on the upstairs socket. That being the case, makes me think that it must be affecting the downstairs line in some way.

With the faceplate removed so the extension is not working take a look at the stats, if they are better then simplest method is remove the ring wire so that the extension has less of an effect noise wise

http://www.coolwebhome.co.uk/calc/index.php

Downstream is improved by exactly 1Mbps! - which is the same difference of downstream data connecting router to upstairs socket (as compared to downstairs).So improvement is 3.5Mbps to 4.5Mbps.

But upstream is still the same 448 Kbps which is obviously the ADSL limitation.

However, pages are loading a little more snappily, so I'd say that's a result.

I'll pull out the green and green and white wires and fold them back. We never use upstairs line anyway. Unless that seems like a clumsy way to do it, by anyone better informed.

the best answer is always a single pair terminating on an NTE5 where the modem connects, with any extensions filtered.

A split ahead of the master is a bridge tap and a bad idea.

Thought as much. Will have to read up on it as I have no idea as to the wiring scheme. Have rewired phone sockets before though. But not NTE5 'out of bounds' demarcation... Perhaps someone can show me a link that illustrates it clearly.