my water won't heat up although the radiators are very hot - when I put the immersion heater on this does not work very well either - when previously it would take 10 mins max to heat the water it is now taking 30 mins or longer and then the water only lasts a very short while and will only fill a couple of inches in the bath before it runs cold. Is this somthing to do with the hot water tank in the loft? does it have a termostat or element that has broken? and how should I go about mending, replacing it - is it easy to do or would I need a plumber? I am a woman who knows nothing about these things and would appreciate some advice.