Roger Mills wrote:

4. Multiply by the declared calorific value (CV - see last gas
bill) to get consumption in Megajoules.
I've always wondered why this is required. Does the calorific value
vary ?

Yes, a bit. Looking at mine for the last 3 years, it has ranged from 39.0 to
39.6 - not a massive amount, but about 1.5%

Am I right in guessing that a higher calorific value means that more
energy is released when the gas is burnt ?

Is this just the gas companies fleecing people, or does it cost them
more to supply gas with a higher calorific value ?