[cb750chris]

Just to add an alternative to Harrys rough pressure drop test, for my caravan I tend to do something similar, I pressurise the pipes (enable the bottle), then isolate the bottle valve and leave for a few hours.
But then instead of trying to hear a hiss as the system is re-pressured, I turn on a hob burner, and check pressure is left in the system - normally after 2 or 3 hours.
The reason being that I can't always hear the hiss as the bottle fills the pipes, but I can hear the gas "escape" when turning a hob on (I prefer to hear a positive "pass", rather than possibly missing a failure)