plusnet evening slowdown

Increasingly for a month my plusnet connection slows to a stop most evenings, from daytime speedtests of 17- 19 mbps.. Plusnet have adjusted some settings but there has been no improvement. Any ideas as to what could be wrong appreciated to help in speaking to Plusnet

Have you run a thinkbroadband speed test? A classic sign of congestion (possibly on the BT side) is where the x1 speed is far worse than the x6 speed.