Redrange,
I don't know but I can maybe help you find out. Firstly, I'd be pretty suprised if it was a switched earth but then this is a relativly quirky Britsh car, most of my experience is with Jap cars.
If you've got a multi meter you can find out though. Take the bulb out and test the bulb terminals with the car in the condition in which the bulb will be off (ie lights off, doors shut, whatever).
The difference between a switched live or earth can be established thus. With a switched live one terminal will make a circuit with the body, the other won't make a circuit with the +ve battery terminal (assuming the car is a -ve earth, most are).
With a switched earth one terminal will make a circuit with the +ve terminal and the other won't make a circuit with the body.
Best to test this using a multi meter with a fuse built in and make sure the vehicle is in the light off mode or you'll get confusing results.