Riello RDB2 oil burner wont ignite when hot?

I have a Riello RDB2 burner in my boiler. After fuel running out and subsequent re-fill the burner ceased functioning about 1 week after new delivery. Water/sludge was removed from tank , fuel line, filter and a new nozzle was fitted. Now, the burner fires up fine from cold and will run for 30mins or so untill boiler stat or room stat cuts burner out normally. When the stat cuts in again the burner locks out and will not start. The fan runs and ignition cycle starts ok on pressing red lock-out button, but no fire. If left to get cold the boiler will start ok on pressing lock-out button, and will run happily for long enough to get system up to temperature again - then on next normal stat cut-out the lock-out happens again on stat-on. Any advice on possible cause?