Thank you all for your responses - much appreciated.
This just highlights why self-install was such a terrible idea. There must be thousands of people with bridge taps, I know my friend had one when on Digital Region as I had to go and remove the ring wire, but I wasn't aware of bridge taps back then so that remained (but didn't impact his service seeing as we were the only two VDSL connections on the cabinet I suspect plus he was probably only ~100m line length).
Well, actually, it was a pure accident that I noticed what I described in my first post; it happened while I was disconnecting the extension. But thanks to this forum, I now know what happened. By the way, my Fritzbox also reported that there was a bridge tap (they called it split connection or something like that).
There was no impact of the bridge tap on our connection apart from what I described above.
In my case self-install isn't a terrible idea but done for pragmatic reasons so the router can be in the hallway while the master socket is the master bedroom; I'd hope that the extension is correctly connected. Certainly no OR engineer said it wasn't.
So, the wiring (unshielded twisted solid core cable) is connected to the 5C master socket (fixed part), while the bell wire is connected to the removable part of that same master socket (as we still have a retro GPO phone). All that to the extension socket which is ADSLNation filter. My understanding therefore is that unfiltered signal from 5C goes to the filtered box (ADSLNation), where it is filtered for the GPO phone and broadband.