heatflow boiler oil leak engineer cannot repair?

we bought a house with a"home fitted central heating system!" rads rusted and leaking water etc, my main worry is the engineer has charged me over £260 over 3 visits trying to repair an oil leak from the boiler(in my kitchen) he fitted a new pump, valves etc, it still leaks! so I refused to pay him the last £80 ontil it was fixed, that was a year ago, he has never been back, he said it cant be repaired, I am worried now about the smell and safety, we are disabled and my husband is a pensioner we do not have anymore money.