I am not an engineer but this is the simple way to get the boiler going again after running out of oil.
Assuming it does not need bled - look at the temperature gauge on the boiler - beside it is a black "knob" - turn it and take it off. It reveals a little white 'thing' push it and that should let you hear the wonderful noise of a firing boiler.
It wouldn't have worked in our case as it's not gravity fed. The tank's about a metre below the boiler, so you have to go through the plastic/rubber pipe from the pump into glass jar and repeatedly start it until the bubbles stop coming...
