The following is a Fire and Gas safety guide produced by the HSE and the LPG association. It is an old guide but the majority of its content and advice is still relevant today.
Gas engineers are no longer termed 'CORGI registered' however but are registered as 'GAS SAFE engineers' - you should still look for this LOGO or their card if you want assurance that they are registered engineers. As you read the guide just replace the word 'CORGI' with 'GAS SAFE.' Download the guide here (pdf).
There are also a number of links or references to additional sources information. Many of these no longer exist, however you can further helpful advice on the LPGA website and the HSE website.