i am not sure of the name of the part but i have had this problem before and am having it again right now ( engineer coming out tomorrow to fix it ) ,the problem has in the past been loss of pressure in a membrane (bag) inside the boiler ,it is there to allow for the expansion of water in the boiler .the engineer usually pumps it up with a car foot pump . job done !
Join the british gas care package and you wont need to worry any more ( i dont work for british gas )
maddog mcg
December 2008