Or you get the weird situation when two or three items have very slight faults and work OK on their own, but trip out the RCCB when they're all on together!
Installed any new appliances recently (particularly devices that work with water - washing machine, dishwasher etc). These are sometimes pressure tested in the factory and can ship with small residues of water. This can be enough to trip the RCD until they have dried-out.
If this is not...
How do you connect the gas pressure manometer to a gas test point? I assume some sort of threaded/barbed convertor is required to connect to the rubber hose?