My Glow worm Space Saver 50 boiler appears to have overheated?

The boiler was having trouble firing up on Saturday morning, this appears to have been solved by securing the 3 port motorised valve. The boiler ran OK until Tuesday when there was an odd smell from the airing cupb'd. On opening the boiler case and removing the bottom plate there was a number of small sooty deposits in the bottom of the boiler. Also the inside of the case and face of the boiler itself had a thick coating of black soot deposit. This suggested the possibility of burning. On looking into the burner chamber I could see a mass of solid soot build up on the underside of the water chamber. I removed all the loose soot and relit the boiler, it ran for a couple of hours but the case became very hot and it shut down including the pilot light - any ideas please?