Problem solved.
Dad installed three Mesh units some weeks ago. Nice wifi throughout the house and garden. Not being familiar with these little discs, I thought they were for wifi only. Turns out they have an ethernet socket on the back. Plugged the BT Box into the closest Mesh AP and Bob's yer aunties live in lover!
Re: Powerline adapters, I have no idea how good they are , but in my parents 130 year old house, rewired in the 80's, they are a bit crap for streaming TV.
For those that offered help, thanks. As for the 'nonsense' remarks, I can only go on the facts before me. BT TV fails, chap comes round, checks the internet, replaces the box, BT TV works fine for 6 months or so, same thing happens again, repeat. After the 3rd box, they stopped sending a chap round and just sent him a new box. Now, it maybe that these returned boxes are coming back out of their workshops as no fault found. I don't know.