Okay. I'd heard rumours of that approach to the gas stuff - nice to know that someone has actually done it. Look forward to seeing the finished version appearing in this forum sometime 
I haven't looked into water meters much, because we don't have one here. I did come across some info whilst I was looking for info on the gas meters though, and it could be that the clunk is the "pulse" referred to on http://www.bellflowsystems.co.uk/Single-jet-Cold-Water-Meter-p-16202.html? Maybe there's an electro-magnetic pulse to accompany the clunk, and that might (or might not...) be easier to detect than listening for the click?