The radiator and hot water circuits are totally different m8, the radiators can be working ok but the hot water side of things could be bust, this is usually due to a faulty pressure sensor in the hot water circuit which senses a pressure drop in the water supply when you open a tap and then operates a divertor valve which re-routes the water flowing round your radiators to a heat exchanger to heat your water.
Easy way to tell whats faulty is to watch the main flames while someone operates a tap, if they go out then the pressure sensor is ok and the valve could be faulty, if the flame stays on then the pressure sensor isn't seeing a pressure drop.
However combis have a lot of electronics within them and some of these could be knackered, thats when it gets expensive and hard to track the problem down.

HTH
Steve