ideal combi boiler c95ff no hot water ?

we have had our plumber out to look at our boiler he said it was the main pcb board the woman at call centre said it was the ignition board to why we have no hot water (the heating works as normal) we have replaced both boards but still have no hot water we can turn the shower or the hot taps on and the pilot light goes off as soon as we turn water flow off the pilot light comes back on please help