remove filter and plug the router/extension cable back into the phone socket, no need for any filter at all.
@Britbrat,
Yes, that would have worked, if I could arrange for the RJ11 outlets as per HP's comments, but even so it can be improved upon...
...if I just connected the router to the phone wires then the entire house's phone extension wiring would still be acting as an aerial and feeding interference into the signal seen by the router. The solution I'm pursuing improves upon that that, such that the DSL signal is only exposed to interference picked on by the wiring between the master socket and the router. The extension wiring to subsequent remaining sockets (at least another four) will be on the filtered side of the DSL signal, and therefor shouldn't be able to contribute much to interference.
Better still would be to filter the house wiring in its entirety at the master socket using something like the filtered faceplate you linked to earlier. That's the Gold Standard solution, but it requires the router to be plugged in at the master, which I want to avoid. All the same, when you think about it, my solution isn't actually any more complicated, it's just the same think but implemented 'downstream' from the master socket.
To put it all in context, the guy who had my house built installed phone sockets in every room in the house and then some (I can think of at least nine sockets off the top of my head, there may be more if I went around and counted them). It probably seemed like a good idea at the time. I could just disconnect it all but that seems a shame, I'd rather find a way of leaving it in place but minimising it's effects.
edit: added 'at least'