Thank you, Dog (and Vxfan). I'd heard that carb cleaner was the thing, but I also remember there was a strong carbon solvent that used to be used commercially to decoke engines, in the days when that was a regular requirement. Can't remember the name, though, and it may have long since disappeared...
I managed to remove the valve - the one on the Arosa/Lupo is water cooled, which probably makes it more prone to furring up, and it was certainly well clogged. Unfortunately, I couldn't separate the actuator from the body, and there's limit to how much carbon you can shift with a Q-tip! A long dunk in a suitable solvent should do it, though, so I'll try again when I've found a suitable quantity. It seems to be stuck in the closed position, so it shouldn't affect day-to-day running, although I'd quite like the light to go out!
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