Have had new pump fitted as was experiencing circulation problems. Slightly better but still some rads cool at bottom so suspect sludge. I will get it fushed. However sometimes after a couple of minutes operation the flame and pilot extinguish. Sometimes the boiler will run for 2 hours. If you watch it happen through the window, the flame pattern becomes erratic, the flame starts to lift off the burner surface and within 30 seconds the gas valve shuts off. Could it just be overheating ? Anyone any ideas please?