Firebird Riello RDB burner photocell sooting up?

I've got a firebird c20 condensing boiler with the Riello RDB burner. The photocell keeps sooting up every three-four days use. This has been going on for over a year. Had 5 engineer visits from two service companies, replaced the cell, stripped the boiler everytime, replaced oil in tank, fitted extra filter in the oil line, loads of printouts stuck to the boiler from gas analysis etc to check its running ok but nothing has worked the cell soots up again and again. What do I do? replace the burner? Any ideas?