Ran out of oil?

Riello RDB burner on a Firebird 90

Ran out of oil the day before yesterday. Got a refill, bled the burner, and after half a dozen resets the burner fired up and stayed on for 7 to 10 minutes. Then it went off and after a couple of minutes tried to fire up again but couldn't and locked out. I presumed there was still a bit of air in the system so went through the process again... same thing happened. Each time I eventually manage to get the burner going it will stay on for 7 to 10 minutes (presumably until it gets up to temperature) but then can't get going again. I've had it going maybe 7 or 8 times over the last two days - each time for 7 - 10 minutes.

Clearly all the components are operational (otherwise it wouldn't fire for 7 to 10 minutes).

I'm thinking that either there's still air in the system, or there's some other problem related to running out of oil... condensation? gunk from the tank having been sucked through?

I must have reset the burner 100 times but given that it says on the burner cover "reset twice only" and reading some of the "if you reset it more than twice your house will explode" posts on sites like this, I think I've gone about as far as I dare!

Next step is to get a professional in... unless anyone can offer any pearls of wisdom?

Thanks in advance.