If its running and you can smell kero, it probably isnt leaking within the burner or combustion chamber. Does it have a 2 pipe system and if so is there any leaks. If its sooting up the pc then its burning incorrectly. What parts did they change. If your in the Lancashire area fell free to give me a call 07881 620285
Paul Cunningham Plumbing and Heating OFTEC, March 2009