It is just a standard festoon bulb - Halfords will sell them. However, I bet the lens has broken where the screw fits in. That is usually the cause of the numberplate light out indicator. The lens drops and the bulb no longer makes contact.
I've had 4 of these go now, and it's starting to hack me off! The number of B6/7 cabrios I see with a number plate light not working seems to support my theory that the design is a bit weak, to say the least!