Paul and Matt have bent over backwards to help me and thanks have to be said.

About the sniffer, apparently looking for the correct frequency sound is the trick (not that I know anything about it - that's what google said) I found a forum thread somewhere talking about a little circuit that plugs into a smartphone and you scan across the surface with a microphone to get visual clues regarding where a tiny leak comes from. The thread runs dead with no results though, but it does sound interesting. My hearing is pretty good, but if my Mrs found out by way of me finding leaks via hearing I would have a hard time explaining myself