Another lockout problem with worcester oil boiler?

I have a 'Worcester' 'Danesmoor utility'- 12/14- oil boiler,
The burner is a 'electro oil Inter B9A.
First noticed cold rad's, checked out boiler, to see the lockout lamp on the control box was lit, pushed it back in, it all started up fine just as normal, worked fine for an hour, when it went off because the hot water was up to temperature and was meant to come back on for the heating, the lockout lamp kicked in again, tried it once more turning it off and on again with the indoor thermostat, "lockout" again.
So I ordered a new 0.50x60ES DANFOSS NOZZLE, when it came I stripped out the baffles cleaned them, scraped off all the interior, hovered out the inside (inside no excessive build up, not black or sooty) checked the "photocell" (it was clean, but rubbed it with a cloth anyway) fitted the new nozzle, the electrode gap is ok and the electrodes are clean, also I cleaned the oil filter on the pump,(was not really dirty).also at this point opened the oil shut off tap, and the oil flowed.
Reassembled, put the heating and hot water on, it fired up as normal and run for the day no problem, sounded ok, just as it always has, thought I'd cracked it ! -- next morning -"LOCKOUT" I have reset the lockout button three times today and each time it run's perfectly, but as soon as it switches itself off -- then, when its asking for a relight -- "BLINK" it locks out again. Got any Ideas ??? The boiler is about eight years old, it has had about five or six new nozzles and about four years ago it had a new coil.
I was thinking that, that would be my next step, a new coil,,, the control boxes insides looks as new as a button and seems to work fine when I'm watching it, but I have thought of replacing that as well, what do you think ???? I am at a loss what to do really !!!!
Anything you could advise would be very much appreciated
Thanks for taking the time to read this
cheers Mark