Megane 55 reg.Heater is on full,but not turned on?

I have a megane estate 1.5 turbo diesel.(55 reg)The heater has come on by itself,whilst my wife was driving,She said it did it a couple of months ago but went off again after a bit.But it wont go off now only when you touch the brake peddle,any help please would be much appreciated before i take it to a garage.