Gas fire keeps "cutting out"

Or it might be an oxygen depletion sensor stopping the fire from killing you.

If it hasn't had a service including a flue check for the last 10 years I'd suggest treating it to one. At one a year, a decade can add up to a lot of birds' nests.

Owain

A "coal effect real flame fire" burning gas and shoving most of the heat up the chimney or a gas fire with incandescant fire bricks?

You could be right about the flame sensor/thermocouple. But usually they wont light up at all if the thermocouple is US. You can get them pretty easily, there are universal ones that fit most things. But if it has never been serviced in ten years there are lots of things that will need cleaning out. It could even be dangerous. The flame could be "lifting off" if the gas/air ratio is wrong through some kind of blockage.