HALSTEAD QUATTRO BOILER - COMBINATION BOILER - PROBLEMS?

Hi - I am having troubles with my boiler - where the hot water would come on automatically when a hot water tap was turned on it no longer does. For the past few weeks, if I switched on the heating and turned on a hot water tap, then the hot water would come on but now the hot water only comes on for a few minutes then gets cold again. Also, when I switch on the heating (slide the button on the boiler to the "on" position), it comes on (sometimes needs a bit of help and I have to push the button on/off a few times and then push the overheat lockout reset button - as sliding the button to the "on" position on its own often does not start the boiler) but only stays on a few moments then the heating stops working. No error message or hissy noise, or red light, the system just stops functioning.
I would be grateful for any advice as to what might be causing the problem. Thank you.