I know this is a long time after the question was asked but hopefully it'll help someone. It'll be the pressure switch. Mine stopped working yesterday out of the blue. Take the front off, push the pressure switch down and I can get hot water but it won't go down on its own. An looking at the website it requires the purchase of a new shower 'engine' as you can't buy that part alone. That's 2 gainsborough showers i've had and they've both broken after a couple of years. Not buying another one.
Colin Henderson, January 2008