To calculate gas usage take inital meter reading from a later one. That is volume of gas used.
If in cublic meters (m3) then divide (÷) result by six (6 as in maximum hourly rate in cubic meters of a U6 typical domestic meter max) and multiply (×) by sixty five (65 as in maximum kW of a typical domestic meter ) to find energy in kiloWatt.hours (assuming Calorific value (Cv) of thirty nine Joules per Cublic Meter for energy density / quality. You can get the actual figure by zone / region at National Gas but that will do)
Multiply the energy used in kW.h by the kW.h cost in pence from tarrif to find out how much cost you.
There are online calculator for day to day calc (but can just use your two readings with made up dates).
The meter will say if in hundreds of cubic feet rather than cubic meters, whihc requires extra step in calc or different multiplication. If meter older than 1996 may be the case.
Bills now in kiloWatt.hours for energy use rather than Therms used to be. Joule is the SI unit but kW.h was used for electricity as could times power in kiloWatt by hours used to get energy use. The Watt (a unit of power) is a Joule (a unit of energy) per Second (a unit of time).
Sometimes when meter upgraded the Imperial Flag is left on Supply Point database held by Xoserve , and also used for billing calculations, so customer end up paying three times as much.
Yet to be added to this page more on Natural Gas utilitisation and demand. For now check out National Energy Efficiency Data-Framework (NEED) at DESNEZ and other links from homepage.