This is starting to get ludicrous now. Your engineer was here yesterday evening, very helpful chap, and quite knowledgeable too!
He changed our socket on the cab, changed the connectors on the outside wall, confirmed the internal setup was all good, switched out the router anyway.
About two hours after he left, we were watching TV in the lounge (the Apple TV is no more than 1 metre away from the router), and the film just stopped playing, speed test from the same room as the router below:

So, this morning, I’ve done a full house scan via Virgin Connect, as well as a couple speed tests. WiFi is showing good signal in all rooms except two that showed as fair (one of which is the kitchen, literally one wall away from the router).
Next to the router I was getting a good 274Mbps, yet the highest in any other room was 90Mbps (directly above the router), the lowest I saw was 15Mbps with the average across the house (including the lounge, where the router is at 274Mbps) was a massive 63Mbps.



The upstream and downstream values from this morning in case they are helpful:


Please can we get something new to try. The fact that the speeds drop to a quarter at best (downstairs hallway) when I’m no more than 5 metres and a walk away from the router (and in the same room as a working pod) is utterly ridiculous.