how can I fix my RSF820/20 COMBI BOILER?

takes a few attemts to light the pilot , but gets there eventually.When I turn the hot tap on the water stays cold and the boiler doesnt fire up .If I then flick the heating switch on the boiler fires up and hot water comes through , but after about one minute the boiler coughs and splutters and goes off , and the pilot light goes out again.It then takes a few attemts to light the pilot again , and round we go.