Getting back to the subject of my original post, I'm probably going to try a Solwise filtered faceplate:
http://www.solwise.co.uk/adsl_splitters.htm (halfway down that page).
...unless anybody talks me out of it before I get to the electronic checkout?
According to the blurb, it differs from most extension-fitting (standard patress, non NTE5) faceplates that Ive seen, in that wiring to the rest of the extensions can be either filtered or unfiltered. I don't think the ADSLNatiuon equivalent allows the former (subsequent extensions filtered), but maybe somebody will tell me if I'm wrong, or missing other choices?
That will allow me a half-way house, where I can leave the router where it is, slightly compromised by being on an extension, but optimised by getting rid of most of the interference from the rest of the extension wiring. It also allows me to reduce the number of filters in the entire house to exactly one.
Getting back to b4dger's comment:
I would plug directly into the TEST socket and record your results. Then compare other configurations with these and if they aren't as good then you know you can improve things.
The test socket actually shows no obvious improvement, though it's difficult to be conclusive as it takes some time to unplug all the hardware and reconnect, by which time the noise levels may have drifted up or down a bit anyway. That's why I'm loathe to use it, as it means sacrificing many coinvenience factors, such as having the router's wifi in the centre of the house, and keeping flickering lights out of the Study which occasionally doubles up as a spare bedroom. But it still makes sense to optimise the wiring as much as possible, especially as I'm on a rather long line (56dB).