Air lock in Riello burner?

Ran out of oil the other day, had some delivered now but the boiler keeps cutting out after about a minute, then it will fire up again automatically after 10 seconds and then cut out again for another 10 seconds and so on. It is the Riello Burner, it has a red plastic removable cover, not sure of model number, sorry.
I was thinking maybe an airlock and if so how do I remove it.

Many thanks.