Sounds like the cylinder thermostat may have been knocked, and is not touching the cylinder any moor. So it is not telling your boiler to turn off when the water is hot enough. It should be positioned approx 1/3 up on your hot water cylinder inside the airing cupboard. Either that or it has been accidently turned up. It should be set at 60 degree's . Then again it may have just packed up and need replacing. they are cheap enough to replace you can get them from B&Q for around �15-�20.