Selgas
From your description of the fault and as I am not familiar with that particular water heater I can only go on my experience with appliances controlled by power packs using HS ignition units. Clearly if the main burner lights and stays alight allbeit for 3 seconds then the cause may well be low gas pressure especially if the water heater is supplied by Natural Gas and the gas meter is located outdoors in the freezing cold. In this instance the fault of low pressure is likely to be caused by the gas meter regulator being exposed to extreeme cold temps and the diaphram in the same will not activate properly. To rectify this problem you should contact your energy supplier to check their gas meter set ( GMS ).
Other than the above I could only add that if the problem does not turn out to be low pressure then the fault is likely to lay with the flame proving section of the electrical ciruit or possibly the control box for the same.
