Thanks for the answers so far

The most recent tech who visited, did begin to discuss the possibility of external interference, although he seemed to want to just get the line speed locked down, rather than trying to identify the source of the problem.
I like the idea of having a better router. I would like to be able to control things like what, and how much, our B&B guests can do with our Internet connection. I also prefer to be able to see the stats, which you could do with a BT Hub 3, but for some reason you can't with Hub 4.
However, I make fairly extensive use of their remote hotspot service (BT-with-FON and BT WiFi-X hotspots) and my understanding is that use of this is conditional on a hotspot being available on your home connection. I am assuming that they will be able to tell if I am no longer broadcasting a FON or BT WiFi-X hotspot and may then withdraw my use of the remote service.
Also, I have been led to believe that BT's Indian call centre can be reluctant to help if you phone in a problem and they see you're not using a BT device.
Can anyone shed any light on this?