Glow Worm Space Saver Mk2 - Cutting Out- No Hot Water?

The boiler was ticking over on the pilot light. After switching the system on/off I couldn't hear or feel any water passing through the pump so I changed it and flow is now good. Fired boiler up and ran for 5 mins before dropping back to pilot light. After it had 'cooled' I started it up again and tried the thermostat - it wouldn't stop the full burn when in the off position. So I replaced it - fired it up and thermostat turned full burn on/off... but only for 5 minutes when the whole thing shut off and went back to pilot light... I'm STUCK!? Any ideas would be very much appreciated! Thanks, Ian.