Bentone oil fire cutting out after 5 seconds,how can i fix?

The heater [Bentone Sterling 40 ST108PL]has worked for 3 days ok, then one evening switched it on, it fired up ok, then it cut-out 5 secs later, so pressed "reset" & it fired up again, but cut out 5 secs later,i did this four times, [by now the water was very slightly warm] then i checked the oil supply to pump-- ok, plenty, i think the flame sensor must be ok otherwise it wouldn't fire-up, is it possible that the "fuel solenoid valve" on top of the pressure pump has an intermittent fault,other than those items i'm not sure what the cause for the malfunction is, thank you to anyone who takes the time to read & answer this problem, Bryan, lincs.