Boiler is a Wocester 280, with an external expansion vessel fitted. One minute the boiler was working fine, the next minute when a demand is applied, (eg. hot water), the pressure shoots up from half a bar to over three in just a few seconds. Safety cut out trips and shuts of burner. Lots of unpleased knocks and bangs from boiler as pressure rapidly drops back down again. I know the expansion vessel regulates the pressure, but having had a vessel fail before, the speed of the pressure change is MUCH faster with this problem. I've tried draining and refilling the boiler in the hope of an air lock, but no change. Any ideas what the problem could be?