does the boiler have an integral timer fitted?
i'd disconnect the timer [internal or external] first to see if the boiler stops firing up
failing that id disconnect the DHW thermistor
if neither of them stops boiler from firing im afraid you will the services of a professional to diagnose the exact nature of the fault because it seems like the safety devices are failing to operate & cut the boiler off which would technically mean your boiler would be classified as immediately dangerous
manwithnoname, May 2011
