MISTRAL OIL FIRED BOILER M1A 50/70 CF & BF?

Our Mistral Boiler ran out of oil. We put more oil in the tank but it is not firing up now. It makes a roar then another sound as though it will ignite, then goes off. We loosened the big nut underneath and water came out (murky at first), then a mixture of water and oil. Eventually it beacame yellow as the colour of the oil that went in to the tank. There still seems to be watger in there though.

Can anyone please advise us on how to prime it as we'd like to fix it ourselves if possible before calling out an engineer.

Thank you
Steve