Whlst in the shower I heard a *pop* and black water started coming out of the head. I turned the shower off and on investigation the rubber face of the head was blackened, as if burned. I cleaned it off and turned the shower back on ( not whille in it ). The water ran but it was cold. Trying it on it`s hottest possible setting the water runs lukewarm at best. It is a Newlec Calypso plus. Can anyone advise please?