So...here's the inside of my old "master socket". I can tell it's the master socket since it has the surge protector. I can see the BT trunk line with the mass of coloured wires...however it seems like the people who originally wired it up when the house was built only used the blue and white wires, and have connected wires from another cable.

Do I just wire whatever is in the original pin 2 to the NTE5 pin 2, and so on? Why are two wires connecting to one pin? Because of the extension?

The cable in the white sheath that the 4 wires that connect to 2,3,4,5 must be an extension cable? This must go to the other 'phone socket we have elsewhere in the house.

I won't be using the master socket to plug anything into through the front of the faceplate. I will be plugging a phone and router into the extension.

So if I install the XTE2005 clone faceplate, I don't do the extra wiring into the rear of the faceplate if I want to use a microfilter between the extension 'phone socket and the router?

The PDF for the XTE2005 says "If you plan to plug your modem/router in to an extension (ie, NOT into the ADSL port on the front of the faceplate) you must run an extension & connect it to the A & B connections on the rear of the faceplate. Blue/White goes to A and Blue goes to B, at the other end Blue goes to 2 & White/Blue goes to 5 on the BT socket.". How does that relate to the wires in my master socket? The extension lead is already there (in the white sheath), which wires are we talking about here? And I guess I have to assume the "other end" is wired up correctly since it's a custom socket and I can't get into it (although the 'phone currently plugged into it works fine so I guess it must be wired correctly). Or is it that I have to run two extra wires between the back of the faceplate and the inside of the master socket?

OK, there are two sets of wires going to each pin as that is how the old sockets worked(no seperate input & output wiring). If you look at the new NTE5 you will see two sets of pins, one set marked A & B, the other 2,3 & 5.

Take the cable that comes from BT(WHITE & BLUE in your picture) and connect the wire that currently goes to 5 to A & the wire that goes to 2 to B.

Now for the extension wiring(the cable in the WHITE sheath). As you have the XTE-2005 you will remove the front lower faceplate of the NTE5 and substitute this for the XTE-2005. If the extension you are wiring is for a modem or router you will connect the two wires that are going to 2 & 5 on the original socket to A & B on the faceplate, you will still need to use a microfilter on the extension. If the extension is for a telephone ONLY then connect to 2,3 & 5 on the faceplate(wire 3 is required in this instance).
If the extension is for a modem/router as well as a telephone & you do not want to use a plug in filter you can use this socket :-

http://www.run-it-direct.co.uk/BTsocketFILTEREDwithRJ11.html

Okay, almost there. Essentially, the blue & white cable from BT (black sheath) goes from 2&5 into the back of the NTE5 (5->A, 2->B), and the extension wires go from 2&5 into the XTE2005 faceplate (into A&B - is it 5->A, 2->B again?).

Thanks for all the help.

That is it

All wired up, no bellwire connected...700kb/s!! Sweet.

Thanks for the help!