Anyway, there was a socket immediately following the block terminal which
I
have discarded as it was is a bad state. I've subsequently wired up my
extensions to the junction box, using standard (slave?) phone sockets.

I don't know much about phone wiring, but it's just come to my attention
that this may have been a master socket, however my phone still works -
would this work if there was no termination resistor & capacitor?

I'd like to replace the block terminal as the contacts are corroded and
the
box is really shabby, what could I replace this with (I don't need a
socket
there)? The box has 3 wires going in - one is my extension, one is twin
bell
wire and the other is 3 core which I think goes to some old sockets
upstairs.

BT own the cable that comes into your house, up to and including the master
socket

If the master socket is a NTE5 socket, as far as I am aware you are allowed
to remove the lower half of the face plate to connect your extensions to.

If you have removed the master socket, the phones will still work, except
one key point
They may not ring!
Older phones are more likely not to ring, where newer ones sometimes will.
The master socket filters out the ring, and feeds it down another wire.

A master socket needs to be reinstalled where the line enters the building
BT should do this, but will probably charge you.
However, it is not impossible to do it your self!

There should be two wires entering your property (Probably the twin bell
wire)

You need to put a master socket here, connect these two wires to the A and B
terminals on a NTE5 socket
(The lower half of the face plate comes away separately to the upper part on
these)
The A and B terminals are in the second part of the face plate.

Then you need three cores running to all your other sockets (pins 2,3 and 5
both ends)
(If you are using a master socket that doesn't have A and B, connect the two
incoming wires to pins 2 and 5)

There is some more info here
http://tinyurl.com/4gu1

Sparks...