A tenant of mine has phoned to say that the central heating oil boiler has stopped! It is still trying to fire when he trys to ignite but he is saying that there is no oil getting to the boiler, even after bleeding it. He has loosened joint after the stop tap at the tank end and there is oil coming out but at the other end of the 10mm supply pipe (before filter or anything) it does not appear! The supply pipe from the tank runs under the garage floor to the other end of the garage and comes up at the burner. What would be causing this? What would be the best ways for me to find the problem?