Many thanks, all. I did the wiring last night and it's working as expected.
I've ordered the faceplate and modules that burakkucat was careful not to endorse, and in the meantime lashed up an adaptor at the socket serving the modem (wires 1+6, remember, so an ordinary BT-to-RJ11 is no good). I also ordered some plugs and will try making up my own modem cable. Using round cable I think that will work OK, and will mean I don't have to mess with the socket.
"Not a lot of point", said ejs and Alex Atkin UK. I agree, in the sense that I wasn't trying to improve connection speeds. I'm happy with my ADSL performance, which hasn't changed with the new wiring arrangement: 20 Mbps sync at 3 dB SNRM. I hope it continues to work fine when I move to fibre.
No, the only point was to get rid of the original dangling microfilter and possibly to benefit from good-quality components in the Mk4 instead of whatever cheap filter my ISP chooses to send me. I originally thought about putting a filtered faceplate on the extension socket serving the modem, but couldn't find such a product.
But having read what burakkucat and tubaman said about bridge taps, I'm thinking there might be another advantage. I can see how a modem plugged into the master socket could suffer from reflections off the extension wiring, but surely that's true as long as the modem is plugged into an extension that is not the last on the line (as is the case here)?
If so, I guess I should try plugging my modem into the test socket. If that improves the connection I could then think about cutting wires 1+6 on the "downstream" side of the modem's usual socket. Since that would allow the downstream sockets to continue working for phone service, it would be an argument for keeping the DSL signal on a separate pair instead of filtering at the point of use.
Charles
PS: although the filter section of the Mk4 has a quality feel, isn't the rest of the NT5c a piece of junk? I can now see why people here recommend sticking with previous versions. The faceplate is attached by the skin of its teeth, and I don't like the snap connectors -- mine were really stiff and it's hard to get both wires in straight. It's as if they looked at Wago connectors and thought "what's the worst copy we can make?". OK I bought it on Amazon, but it has Openreach branding and looks genuine.
I made the mistake of pulling the faceplate apart so that I could look at the filter section. What with the socket covers dropping out and those long pins on the "bridging" plug, it's hard to put back together