I currently have the same problem with a Staxgas fire.
The problem is usually lack of gas pressure at the pilot light'.
The three flames on the pilot will be reduced ,the one
on the right side will not be making good contact with the thermo-couple pin..when this pin is not hot enough
it turns off the gas to the burners..The solution is to clean out the small pipe and jets for the pilot light...if this doesn't work ,you may require a new pilot light assembly which costs about £45/50.
Ron Campbell R., January 2010