Radiator's fine but no hot water?

I've a "ICI Caldaie Solar 80E"combi boiler,

Radiator's are working correctly but cannot get hot water.

Hot water last's about 10 second's and then boiler goe's off.When I leave the hot water tap on..the boiler will shut down and fire up in interval's of approx 10 -20 second's.

Any guidance/help appreciated.