I've attached photos of the master socket. It looks like the ring wire has already been disconnected.
As for the extensions (there are two in the house), they are both wired into the back of the master socket.
The extension I'm using was fitted probably when the house was built in the mid 1980s.
As of this morning the router stats are as follows:
ADSL Link Downstream Upstream
Connection Speed 6578 kbps 440 kbps
Line Attenuation 42 db 13 db
Noise Margin 9 db 19 db