@Andy_Wright
Andy, you need to understand that your gas meter gives you a usage figure.
Look at your gas bill to find out your actual usage.
There is a formula supplied to work out the Mj you use.
I don't has gas and never had it in a rental house.
When I started looking into USER BILLING, I got copies of bills from clients for Elect and Gas users.
Six year later, IT LOOKS LIKE AGL IS JUST MAKING UP BILLING at the moment.
Good LUCK