boiler not staying alight for long when hot water switched on?

when the hot water is switched on the boiler lights ok but only stays on for short period of time. water does get hot in the tank upstairs but noticed the green 'below temp' light on the tank thermostat stays lit, even when thermostat set at lowest temp of 45degC. The central heating seems to be working ok. suspect the tank thermocouple or thermostat. Is there anything else it could be?, and does the potterton combi boiler system have a safety feature that turns it off and stops it from just heating the water constantly?. many thanks.