Is it a combi boiler, popsy?
Mine has a flathead screw valve (as opposed to one with a tap on it) in a water inlet pipe - you turn the screw 90 degrees, so the groove in the screw runs along the pipe not across it, IYSWIM? That lets water into the boiler when the pressure drops - you have to watch the dial and hold the screwdriver in the screw so you can turn it off again as soon as you need to (have to be careful not to let too much in.)
That's on "the pipe leading up to the boiler" that momof2's DP mentioned. Mine is 20 years old though, yours may well be different...