Started my "tidying up" process.
Preparing to excavate a hole in the wall for a 16mm back box that I "liberated" from my old BT/ADSL line master socket.
Meanwhile the router is connected to the Test Socket via short leads including an RJ11-RJ11 connector (as noted in post #4); speeds are slightly better if anything than via the VDSL faceplate.
And I've got a "simple" phone-only faceplate on order, when that arrives I'll think about the option of a "captive" RJ11 lead wired to the backplate telephone extension connectors.