Worcester Danesmoor 12/19 Oil Boiler Trip?

After several minutes of running the boiler trips. Manually resetting allows the boiler to begin it's purge cycle but trips before this is complete. Resetting again allows the boiler to complete a successful purge then fires. Firing lasts for several mins then trips. This continue to happens. I have changed the control box, checked and cleaned the pump and filter. Changed the ingnition tranny ( although this was ok) Changed the electrodes and cleaned the nozzle. When running the boiler I have removed the solenoid ( oil feed to pump) and the flame goes out ( obviously) the boiler however continues to run regardless. This seems to me that it's not a loss/blockage of fuel that is causing the trip. Checked the flame photo cell and believe this only checks for ignition and initial flame. Removing this once the boielr has fired has no effect and boiler still runs.
So.....what next?
Going to check replace the baffle thsi weekend but that's because I don't know what else to try.