I just wanted to post this in case anyone else has a broken radiator as it might just save them a pretty penny on plumber bills.
One of our radiators was stone cold despite the others working OK. I did a google search and found out that it was something to do with the TRV (thermostatic radiator valve), the thing that has the numbers on it to indicate the heat strength. I took it off, wiggled the pin about that was sticking out and voila its working again!
It would have probably cost us about £100 on a plumber call out fee as well