James - thanks for that; I am truly embarrassed to admit to it, but it could well be that I have never let the tank empty sufficiently for the light to come on. (I have only had the car for a couple of months). Now that I re-look at this photo, presumably the red "dot" is the light?
I'll put a plastic petrol can in the boot (/trunk) and let the tank get lower to see if it works ...
Julian