well first you need to find a gas safe engineer to service it
it does sound like a thermocouple problem but normally when they go the gas shuts off so you might get a slight wiff of gas smell but not very strong
it worries me that unburnt gas may be escaping and the possibilities of carbon monoxide due to partially burnt gas is also on my agenda
if you have no carbon monoxide detectors in your home its a very good idea to buy one as these are a life saver
Dave