In case this is any use to others struggling with 'buffering' and connectivity issues with Powerline Adapters / Network Connectors:
Here's a summary of where I was up to until this afternoon:
I re-contracted with TalkTalk and upgraded my broadband to 'Fastest Fibre' (67Mbps) at the end of November. I had been using a pair of TP-Link powerline adapters to connect my TalkTalk TV + Box (in the living room) to the router (in the dining room, near to the FTTC phone master socket). I was still getting some buffering issues when streaming. I uninstalled the relevant Apps on my TV and re-installed them: This helped, but didn't fix the problem. I contacted TalkTalk support by 'live-chat' and arranged to order a pair of TalkTalk Network Connectors in case the old TP Link powerline adapters were the problem. I installed them and 'paired' them. The 'streaming' seemed to be mostly working fine. I noticed that the powerline light was flickering red for a few seconds and then settling down to green for about a minute, before flickering red again. The TalkTalk Service Centre 'report' on 'Connection in your home' showed that the TV 'sometimes struggles to connect to Wi-Fi'.
The 'Live-Chat' people were very helpful and offered to get me some more Network Connectors.
Then I had a 'lightbulb moment'.
I was following this set of instructions to connect the router, Network Connectors, TalkTalk + TV Box and my Samsung smart TV: Setting up your TalkTalk TV Box - TalkTalk Help & Support
I was connecting the Router to the TalkTalk TV + Box using a pair of Powerline Adapters / Network Connectors and then connecting the TalkTalk TV + Box to the smart TV with HDMI cable. This is what may have been causing the problems: TalkTalk's instructions may be wrong!
I tried connecting the ethernet cable from the Network Connector near the TV directly to the TV (HDMI cable between TV and TalkTalk TV + Box).
BINGO!! The Network Connectors gave a full set of green lights. The TV and the TalkTalk TV + Box still work together properly and the streaming works with no suggestion of any 'buffering' issues!