Connecting powerline adapters via an extension cable is a definite No No , even more so if it's a surge protector one.
As for cable , certainly don't bother with Cat 6 or 7 as these can be inflexible. Cat 5 or 5E is more than adequate. I think the cable supplied by BT was Cat 5 initially and that was OK.
Can you check your internet speeds as shown by the HomeHub. Can't tell you where to find them in a HomeHub as I've never used one, but would be handy to be sure that there's no problem with your internet.
Yes, Cat 5 cable.
Power Adapter plugged into extension cable in room with the TV has worked for many months so don't know why this would change and stop working.
BT Hub check gives 3MB.
Well a 24meg (3MB) line should be fine.
I do think you need to do 2 things.
1. Have the adapter plugged directly into the mains socket
2. If that makes no difference then try a long ethernet cable to rule out the powerline adapters as being at fault.
Things can , and frequently do , change for no apparent reason but what that reason is in your case is not clear.
It's really too far to try a long ethernet cable. It would be huge. I have one that is long enough to cross one room and storing it takes up a lot of space. I'd need one that went across 3 rooms!! Using the power adpators is a solution to that! But I can eliminate the extension and plug into wall socket instead.
Sorry been busy with family-related stuff etc and having time away from the BT Community Forum's.
Does you YouView box still buffer? How many devices are connected to you BT router, because if people / device (s) are connected over the same powerline adaptor (that's if the adaptor is WiFi) and how fast is you're internet speed?
I 100% do feel your concerned about this & so would I be 🤔🤗.