There are several online calculators to convert readings into energy usage and so cost.
To calculate gas usage, subtract the initial meter reading from a later one. That is the volume of gas used.
If in cubic meters (m3), then divide (÷) the result by six (6 as in the maximum hourly rate in cubic meters of a U6 typical domestic meter max) and multiply (×) by sixty-five (65kW is the maximum energy flow through a typical domestic meter) to find energy in kiloWatt.hours (assuming Calorific value (Cv) of thirty-nine joules per cubic meter for energy density/quality. You can get the actual figure by zone/region at National Gas, but 39 will do.
Multiply the energy used (kW.h) by the kW.h cost in pence from the tariff to find out how much it costs you.
The meter may read in hundreds of cubic feet rather than cubic meters, which requires an extra step in the calculation or a different multiplication. If the meter is older than 1995, that may be the case.
Bills are now in kiloWatt.hours for energy use rather than Therms that used to be. The joule is the SI unit, but we use kW.h for electricity because it multiplies power (kW) 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 a meter is upgraded, the Imperial Flag is left on in the Supply Point database held by Xoserve and used for billing calculations, so customers end up paying three times as much as they should.
Find other links relating to Natural Gas utilitisation and demand on this site and elsewhere. To compare your usage with the typical for the same type of household, check out National Energy Efficiency Data-Framework (NEED) at DESNZ.