Let's Fix The Annoying Mains Hum On Our Audio

I've always had problems with mains hum in Tripplikit's workshop / lab / studio. This is not surprising since I am trying to do audio, computer and electronics work up here. However the hum has been really starting to annoy me so lets see if we can cure it.
I started looking for earth loop problems but in the end found the worst of the hum was coming from a completely different source.