Yep, decided to re-arrange my schedule and visited this afternoon.
So the image above is alledgedly the only socket in the house.
Ref the bell wire - on pin 3 - I removed that after the photo was taken. Yes MrSaffron, green.
Ref the second pair going on to an extension - no idea where that is, but I can easily disconnect it. Not to worry about pulling the wrong wires - I've got an IDC tool just in case.
Some info - cable comes into property into an old junction box - see pic with crimps. https://ibb.co/d3uXSA
Then shortly thereafter through another junction box - see pic. https://ibb.co/kVzfxA
It seems to me that effectively there is the one cable coming in, then one cable going out. Which goes to the above "master socket".
The "master socket" does have a BT logo on the front. https://ibb.co/gWY3Zq
Here's a slightly better pic of the rear of the socket. Taken after removing the green wire from pin 3. : https://ibb.co/k1q3HA
So, we need to organise to get that updated to a modern proper Master Socket.
Additionally, I got him a new router. I was kinda hoping to plug it into the test socket crossing my fingers and magically maybe we'd get some sort of improvement. Hoho.
Anyway, XRaySpeX - we'll leave it alone for a week or so, hoping that DLM relents and we see some improvement on that score.
The stats from the new router are in this pic : https://ibb.co/dBfLxA
A slight complication - but I'm not going to worry about this for the moment. Priorities. The PC and location of the router are upstairs. This is fed by a long extension phone cable going up the staircase etc etc. When I installed the router there, it would not sync with a filter fitted. I had to remove the filter to allow it to sync. We'll deal with that when everything else is good.
NB - in case I'm confusing anyone - all the above pics and tests were run right beside the "master socket". It was after that I moved everything upstairs.
So, in a nutshell - then plan is (1) replace the socket , and (2) leave everything untouched for the next week. Hopefully number 2 will allow DLM to relent and maybe sync will improve.
Thanks to all who have contributed so far. If there's anything else I should be doing, will appreciate the nudge. And I'll update this thread when there is any change.