hi mine drops regularly but i was advised by my plumber who is close friend exactly how to gain pressure. there is a screw underneath boiler case that if you turn it whilst off at the mains and positioned off on boiler can be easily turned till pressure goes up, its slightly left offside and requires flat head screw driver about the size of an old halfpenny, when presure is gained turn back until you can,t hear pressure filling up anymore.
pam f, January 2008