Andy, We also had this problem with the same boiler. Our plumber also changed a membrane thing that responds to pressure when the hot water system demand is on as well as replacing the flow switch.
The rubber had started to perish - ours is 3.5 years old so you may need the same. This cured the problem for us.
Sorry I'm not technical and can't remember the exact name of the replaced part.
Regards
Sorry I'm not technical
Keith
January 2010
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