As I have mentioned the problem is with a big drop in internet speed in the evening (but not every evening).
I have tried connecting the mini box via wifi and it kept dropping out so that is now back to ethernet and I have swopped the main box to downstairs and the mini box upstairs which is plugged into the router. The other night I turned all devices off apart from the virgin equipment but it still dropped down to 100mb from 210 during the day.
I am now going to leave the set up as is (mini box upstairs connected to the router and tp link, main box downstairs connected to tp link) and see what happens.
So nothing defined as to why the signal strength drops but I will put up with it so long as the TV works (no buffering and recordings are OK) and internet during the day for those of us that are still working from home.
Lets see what happens.
All that the engineer could do was check the connections said all was OK and changed the connection from ethernet to wifi, which I thought was strange. Now its back to ethernet.