Testing with a hard wired connection is a great way to rule in or out any powerline adaptors.
The powerline adaptors can work very well, although there are things to take into account, for example they should be on the same ringmain. I say this as you mentioned running a cable downstairs.
If you're still getting issues after you've tested with the wired connection, please contact me if you need any assistance and I'll do what I can.
Being on a different ringmain is very likely to cause this type of issue.
I wonder if there is any chance of moving the hub nearer - am happy to discuss possible options with you if you wish to contact me direct with your details.
Whilst it's prefered to have the hub connected to the master socket, it should not be a problem connecting it to an extension.
If you were to do that, would you then be able to hard wire the connection to the BT TV unit or would you still be using powerline adaptors albeit on the same ringmain?
Might be quicker to talk directly with you if you would prefer then I can understand your set up a little better.