I opened up the rear section of the main socket and the feed wire was easy to spot. However there is so much other stuff squeezed in there that I got a bit worried about touching it.
I forgot to mention I have a monitored burglar alarm. There's an outside chance it is maybe patched into one of these extension cables (as the control panel is very near the master switch) and there seems to be too much wiring in there for a BT feed-in and two extension sockets, way too much.
This pic only shows what you can see easily - there is a bit more wiring stuffed inside the wall recess:
The alarm people are coming round on Monday. I think I will leave this to them to tell me what needs to be done...
Thanks so far - I'll post an update.
The alarm people should rewire the extension so that it runs from the back of the bottom removable section, as that is filtered.
They should know where to connect it.
As it is at the moment, there is a good chance that the alarm circuit and other extensions, are reducing your broadband speed, as they are not wired to the filtered side of the master socket.