johnk's right, you've got an air lock and, believe it or not, it's in your cold water pipes. Try turning your water supply off at the stop cock then turn on all your cold water taps and leave them fully open, flush your toilets and run one of your hot water taps just for a minute or two, best one's the bath as you'll be upstairs when you turn them all off again. This allows the ball valve on your header tank to drop and the system to drain down. Then, turn your stop cock back on, about half normal to start and, make sure you do this in the right order or you'll still have the air-lock, turn your taps off starting with the lowest on your system working up to the highest when water starts to flow out of them. So, it could be your kitchen sink's first then your bath, wash-hand basin etc, allowing any toilets cisterns in between to fill before you turn off the next tap. This should force the air out of the highest tap/valve. Once you've turned off the last tap you can open your stop-cock to normal but give a tiny turn back just so the valve doesn't jam open as they sometimes can. Hope this helps you