I have spotted a lot of issues with the Netgear router and BT TV services so I decided to unhook everything, sort all my hubs and rewire the lot. It turns out that my primary homeplug was connected through a daisy chain of power adapters! Rewired everything in the house so all powerline adapters were mainline connected and hey presto... working HD and 4K. The system doesnt freeze up when playing content either.
I use the BT Whole Home WiFi as well and any device that was connected through that WiFi would crumble under the data load when HD was playing, this now doesnt happen so there must have been a bottle kneck through my gigabit switch/homeplug combo that was killing everything.
Happy with TV again! Thanks for everyones responses.