Can anyone tell me how to unlock a car door when the car battery is dead. I can access the boot so I was wondering whether there is a central unlock there? Many thanks.

There is no central unlock
If you can get in the boot you can use a battery charger on the boot light to allow you to unlock the car.
Or you can use the emergcy unlock procedure here
http://www.bmw7resource.co.uk/forum/index.php?topic=449.0