Right! I've fixed it... I think! Was failing at 200mb max last time, currently on 2gb.
We recently got a new modem/router all in one thing from Virgin. Because of the setup here, I didn't want to use it as that, I just wanted to use it as a modem (Would have meant buying a switch for upstairs if I wanted it to be both).
I just connected it up and left them working as they were. Now this happens and starts annoying me greatly, I look into the settings a bit. Found out you can actually turn off the router side of things on it. Great! Did that, now it's working!
Guess some sort of clash was going on. But there we have it!