how can i repair my worcester 26cdi combi boiler?

the hot water and central heating are overheating and shutting the boiler down. when iether are called for on selector. even when thermostat is set low on wall stat for central heating, and the water temp guage on panel set low for hot water.the water temp gets so hot and bangs inside and shuts down gas then the two red lights flash on panel [ c h and h w demand ] they will reset when water temp cools down, but exactly the same happens again . water pressure on guage 1.5 bar but having to regulary top up [ once a month , but we do use boiler constantly ] any ideas gratefully welcome. many thanks.