Local Citroen specialist confirms that it's 100,000 miles.
I had emailed Citroen to ask them, and someone from customer services has just phoned to give me the information. I'm surprised they took the time to phone rather than just email, but very nice of them to do so.
Anyway they said 100,000 miles normally, but if your car is mainly driven it town or stop/start driving where the engine doesn't get to full working temp then it should be done at 80,000.
How much is the fish?