pre heater tank

jessmist

i have a baxi 105 combi boiler with a pre-heater last week i had intermitent hot water i have a service contract with the gas board and the engineer came out and said it was just the valve sticking hot water works now but i never hear the preheater kicking in and hot water seems to take a while to come through is there anyway i can tell if the pre-heater is working by looking at the boiler as i don't want to call the gas board out unnecessaraly and lose one of my discount call outs