Combi failure. No hot water or C.H. ................?

It could be something like a faulty relay in the circuit board which is making the boiler cycle on and off.

The only way to find out what the actual fault is would be to get someone in to have a look.Don't phone a plumber,phone someone who specialises in heating,has fault finding experience and is Gas safe registered.

foobix has the right answer here - a faulty circuit board can make it keep starting the ignition sequence and intermittently play up.

could even be the fan or air pressure switch that means it wont go any further in the ignition sequence and then restart to try and clear the fault.

does it come up with a fault code or a lockout light? what make and model boiler is it? if you just left it with a demand on would it just keep going on/off

and as foobix just said - dont get a plumber - they are trained to repair pipes, a heating engineer is trained to repair boilers! there is a difference!

Is the thermostat turned up or has your pilot light gone out. On the boiler there are two dials to turn one is for your hot water and the other for your heating, turn them both to about 1 o'clock just imagine the dials are a clock face. Now try it.

Is there a flashing red light on the boiler that could mean the pilot light has gone out, if it has been blowing a gale recently that could be the problem.

I was going to say the same as D. Mine was doing the same til I turned pressure the tap on the pipe near boiler. Once pressure reached 1.5 I tried again and it worked.

check the water pressure it should be between 1-1.5 bar sometimes if there is a pressure drop the boiler does not fire hope this helps