Found something out today, that I didn't know about.
I had the boot open for perhaps an hour, whilst I was doing some work nearby.
When I came back to the car, the boot light was out.
I shut the boot lid and reopened it and the light came back on.
So. it seems that the the Mk4 at least, has some form of timer built in, to switch off after a predetermined time.
Can't find any reference to this in the on-line manual.
It may of course be a function of the LED light, that I fitted previously and not part of the standard kit?
Does anyone know??