Speed woes on a new upgrade

[#1]

I've just moved from ADSL to 80/20 fibre, probably one of the  first on a new cabinet, about 350m away with a 67-80mb/s estimate. Sure enough on day of connection the sync was in the high 70s but I was capped by the PlusNet Line speed being at the previous ADSL speed of 12.7mb/s. A quick online chat fixed that.
However I found that speed tests were giving a very variable result - from 40 to 60mb/s as they run. The Hub 1 is connected to the master socket and then to the PC over decent AV600 homeplugs. Even though these should easily be good for 100mb/s I assumed my end was at fault until at around 11pm the problem evaporated and I could download at a very constant 74mb/s
Tonight at first I got a high rate, but then after 7pm things started nose-diving - and my latest BT speedtest is 28mb/s - 45 down on late last night

[#2]

I've got exactly the same problem - between midnight and 6pm, I'm running at almost full speed (73 down, 18 up) but between 6pm and midnight the speed drops.
I've tested 5 times within 10 minutes tonight at around 8pm using the BT Speedtester and the speed has fluctuated between 2.3 and 65 down, with up being a fairly constant 13 to 15.