No hot water & central heating - Sime Super 90 combi boiler?

Combination boiler is 9 years old, no hot water or central heating. When hot tap turned on switch clicks but pilot light & burner do not ignite, when CH put on same fault occurs. Intermittent fault,will be OK for a day & then not work for a couple of days. Diverter valve changed 3 years ago for similar fault, could it be faulty diverter valve or something else.