Hi my name is Brian, I have been in the Plumbing & Heating trade for approximately 15 years, lets see if i can help. do you have a Riello RDB burner on the with the red plastic shroud. if yes then your hexagonal nut is the one your looking for. 4mm allen key i think, loosen the head a little put a cloth under the nut and push the reset button the boiler will go through pre-purge then try to fire then lock out, each time you do this you will get either air or a mix of air and oil out of that nut, eventually it will be more oil than air, shut that nut down and with luck its done. If it locks out do it again. Riello Burners are terrible with air they dont like it at all, .good burner though.
I hope you got it going ok Best regards Brian