right boiler make and model whould help but 99 % shure it will be the diverter valve. its a big washer that goes up and down every time you open a hot tap and in the end it splits and stops moving under pressure which in turn stops pushing a pin up on to a micro swithch which tells the boiler to fire up.
If its just a washer needed these cost about £2 to £3 if you can get just the washer for your boiler as some you have to buy the hole valve which cost about £100 + then you just need to get it fitted.
hope this helps dunc