Baxi 105e hot water problem - runs hot and then cold?

Washing the dishes I discovered that the hot water runs and then you can hear the boiler shut off (the LEDs start moving down) and the water then runs cold... then the boiler lights up, heats the water... then powers down. It will do this continuously for as long as I leave the tap running. The central heating aspect of the boiler is unaffected (luckily!)

Any ideas?