If it's sooting up the photocell on a regular basis it will also be sooting up the boiler flueways and will pretty soon need another service. Who carried out the service and how thorough was he/she?
Did you get flue gas analysis details on the service report and if so what were they?
What make/model of boiler is it, what make and model of burner is fitted and does it have a balanced flue or simply a low level discharge flue?