I heard that some people like to change just one 
GSF supply them singally , although i'm silly i'm not that silly!!!!!!! I already flushed the rad, checked out feeds, changed switch etc, just got in from doing the thermostat after working in Peterborough, stat was knackered but now fans are kicking in and all seems well after getting rid of one hell of an airlock. I have changed all the discs and pads!!!! 