Thanks for all the feedback. I have just bought this car and seeing that the rear diff was leaking (a lot, not just weeping)worried me as I wasn't sure how much fluid would still be left inside.

The previous owner relied on his mechanic at the dealership to tell him when something was required and I guess this was of no importance! because he was surprised when I pointed it out to him. Then again the dealership told me that it was a sealed unit and that a fluid change was impossible!

Anyway, thanks to everyone's help on this forum, I've managed to confirm my suspicions that the dealership was out to lunch. I've changed the fluid myself. I had to buy a 14mm allen key. The bolts were tight as expected so I sprayed them with penetrating oil and drove the car for 5 minutes to warm up the diff. That did the trick and they came out. There was about 900ml of fluid left instead of the required 1.3l so I hope no permanent damage was caused.