This morning my Vokera combination boiler made a thudding sound as I turned on hot water - I went to look at boiler and saw that the water pressure gauge had dropped from its normal 1-1.5 bar to nearly zero (in red zone!). I have looked high and low for a small manual I have but can't find it. Is there something I can do myself to get the pressure back up or is this something that may be wrong with a part in the boiler? Any suggestions I'd be grateful for - I'm pretty capable of mending something if I have some directions... Thanks in advance....