if you view the list of available networks before 21:30 and after is there some new networks appearing? since only thing can think of is somebody next door has a powerful wifi equipment causing to much interference on yours
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
Could be a failing power supply. I had a similar issue when the power brick started to fail. What typically happens is that the windings in the transformer start to break down and it starts to undervolt.
SH2/SH2ac PSUs are not of great quality and are prone to failing this way. The LiteOn PSUs for the H3 are more durable (and compatible). The downside is that they are twice the size and in not quite as convenient a form factor.