The pilot light lights up while you are holding in the gas valve button and, once alight you continue to hold the button in for approx 20 seconds then slowly release the button on the gas valve.
If the pilot light then goes out – the fault lies with the flame failure device known as the thermocouple which will require more than likely to be replaced.
Whilst this is not a highly technical job to undertake it is best left to an expert as the problem may well be the pilot light itself.
Hope this helps – but please employ an service expert to assist you in this matter.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians