Natural gas content varies by places/wells. This applies even in the production line, which may vary by countries. The authoritative source for energy density would be from the producer itself. So, if you're running an experiment, the supplier of your natural gas should have that information.

Thanks a lot Shaharin, and I appreciate the notable diversity of properties that an organic product can have,

However in the same way that a 99.99% pure hydrogen has a set of uniform properties, is there a source that reports on 'standardised' properties of natural gas?

There is no 100% reliable source, it varies from place to place and state to state.