My Potterton 16-22 with similar problem. redhot pipes rads & boiler blasting water into the header tank splashing out soaking the loft floor. Running normal today after changing the thermostat. after a lot of thought I decided the stat was the only temperature/safety device on the boiler.
My boiler starts flames & heats for 2.30 minutes then stops for 7.31mins to dispel the heat, I feel sure your boiler should stop itself if it gets to hot. pump running or not.
Jim R
December 2008