hi,,
your problem relates to the insufficient air cusion in the systems expansion vessle located at the rear of the boiler.
you will need to pump in more air via the vessles shrader valve, pump in air with a foot pump untill your system gauge reads 1.5 bar or in the green sector if your boiler has one.
this should cure the problem.
this procedure is normanly carried out at a service interval at the boilers annual service....
hope it helps.
terry ellis
December 2007