no DHW on my halstead combi 82?

The central heating works fine on my boiler but does'nt fire up on turning the tap for hot water.
Iv'e changed the thermistor for DHW but it made no difference. The pin in [I think the flow control unit] does'nt travel far enough to operate the micro switch,does this mean the flow control needs replacing? sometimes it does transfere the light to the DHW light and then heats up the radiators and then there is hot water if the rads are very hot.