Faz - the motors that control temperature can sometimes smash/break the little gearcog that adjusts the heat of the air, if you have climate control then i believe it's a case of looking into the drivers footwell, into the rear-right of the console, there's a metal palte that the motors are bolted to.
Anyway, unbolt all that, you;ll the cog, then something like a cable-tie/circlip something like that around it and put the motor assembly back on sorts it right out. On mine w/o climate control, i glued the adjustment arm to the dog so it'd turn.
It's possible as you turned it up from cold to hot, it slipped and split the cog. If you have a search about for heater problems on the forum - you'll find the topic widely covered, including one post with pictures. 'Bendy drillbit' should find you that.