Worcester Heatslave Combi 20/25 lock out

Intermittent lockout. Have renewed HT xformer, photocell, solenoid valve, Satronic controller. I've stripped and cleaned Danfoss oil pump, burner head and nozzle. I've checked the electrode position. I've fired the burner outside the firebox and the spark is good and the flame shape looks ok. The only thing I can't check is the filter on the oil tank because the filter bowl bolt is seized and I'm too chicken to risk shearing it off and spilling 300+ litres of oil on the drive. Is it possible that when the pump is sucking, the filter will clog and reduce the flow? Oil does flow freely from the feed pipe at the oil pump.
What have I missed? Is there another control input to lockout that I've missed? Please help before I become just another suicide statistic

You missed spending £10.00 on an oil pressure gauge, thats the best way to see if the oil is flowing ok and if you stripped the pump down you will have prob adjusted the output . you cant clean an oil nozzel it should be replaced at least once a year as the outlet is half the diameter of a hair so any cleaning will just start to block the hole plus if you stripp the nozzle you will see a small cone that spins behind the face this can ware.
basicaly you need to set the burner at the corect CO2 ,oil psi and zero smoke reading before you start throwing parts at it plus make shure the air intake is clear and no oil in the oil (draw off 1 ltr from the burner flexi into a clear bottle and check for water droplets in the base .

a pump can cause intmittent lock out, and as you havent changed it, it must be suspect. as cog says , you need a test gauge. in fact test the old pump first with test gauge.

turn the oil off, before you take off the filter. it will come off.