The car had been losing water for a while.
When I took it in to a garage to diagnose the problem, it turned out there was gas getting in to the header tank system (this was diagnosed by putting a lighter to the header tank and seeing the air inside burn quite well!).
As a result of this I decided to get the head gasket changed to be on the safe side (as I thought a leak from the cylinder to the water jacket would cause LPG to blow through, and the car had done 98000 miles so thought it would be this).
Unfortunately this did not solve the problem, so I trotted on down to Autogas99 / Warrington Autogas's new premises, where they changed a seal inside the exchange unit and told me that the water I put in the system was too hard and had caused the seal to fail.
View the whole blog here http://lpgdreams.blogspot.com/