Alan, it often helps to give a few details about the make and model of the boiler - what happens if you make it run for each service on its own and also both together - by adjusting the programmer / timer.
The general case is that the boiler fires to heat water - an external valve diverts the water to where it's needed - if your boiler fires to make hot water then the boiler is good .
The external valve has a motor inside the box - often in an airing cupboard - the motor drives the valve and the valve makes a switch to fire up the boiler - if the motor is defective or if the switch fails then you'll get no heating - you can often operate a lever on the box to set the valve into the c/h position.
Good luck..., November 2014
