Without a fuel pressure gauge and a Flue Gas Analyser you are at a disadvantage.
However you have checked that the Photo-cell is clean but you can also connect a multi-meter to it and then shine a light into the cell checking that the resistance varies with the amount of light. No light = extremely high resistance and with light = lower resistance. Give the wires a "wiggle" to make sure that you do not have an intermitant open circuit.
You have checked the fuel filters and they are all clean with no ice in? Now get a container. For a single pipe system. Turn the oil off and disconnect the fuel input hose from the fuel pump. Put the end of the hose into your container and turn on the oil. (You may need an assistant) You need a full free flow of oil.
For a twin pipe system. Turn the oil off and disconnect the fuel output hose from the fuel pump at the end away from the pump. Put the end of the hose into your container and turn on the oil and run the burner (You may need an assistant) You need a full free flow of oil.
When you switch the burner on does the flap over the air intake open properly? Now take off the fan housing cover and make sure that the fan impellor and all of the airways are clean. Also remove the blast tube (around the fuel nozzle) and make sure that the blast tube air slots are clean. If the ignition components near the fuel nozzle are dirty then clean them with methylated spirits. Check the ignition gap is correct in accordance with the Riello manual. ( You can downlaod this from the Riello web-site.)
Also, when the burner fires, is there an ysign of soot from the flue?