trianco oil boiler tro 12/14

Seem to have developed a fault on my boiler in the last couple of days,Typical now its gone cold!. Boiler fires every time, runs for about 4 mins and then shuts off(flame only, fan runs on for 15 secs then stops)and then the lockout light comes on. Wait for a minute reset the lockout and the boiler will fire and run for another 4 mins. Would appear to be fuel starvation?,but the tank is 1/3 full(cleaned and checked the sight gauge)cleaned and refitted the bowl filter and plenty of fuel runs through fitting. but the system does appear to have a none return valve fitted, but this seems? to be okay.
Any suggestions hints or ideas would be gratefully received,before we start freezing up.
thanks marko

Check resistance of the solenoid coil immediately after lockout it should be a few K!!

having looked at the solenoid coil found that when the boiler locked out thre was no resistane across the coil(i.e. open circuit)when the coil coled down the resistance came back upto 2.8Kohms. new coil orderedand shold be here on friday
Many thanks for the sound advice, very grateful
thanks marko and family