I have the same problem, it would seem that one of the change over valves on the the circulation unit(cant remember correct name, but is the large brass assembly,located at base of heater) has stuck.Arriston supply a kit which includes a new diaphragm and sealing ring to solve the problem. I have such a service kit on order and await arrival from my local agent in Chelmsford,hopefully this will will resolve the problem of my boiler pulsing on/off and minimum hot water at taps
Keith,Chelmsford, October 2009