Sounds like water pump slipping (might be fine at idle but slipping with a few revs on) to me Ben, if it's left to idle does it get hot quick or only just when driven?

Mini-me (Mr. Bishop) had a similar prob in his old Zetec-S Fiesta which would only get hot on the motorway, trundling around it was fine. Water pump cured it.

Hope you're ok anyway dude!