Hi Dirk, this happened on my '99 Mini facelift Elite a couple of months ago. On mine the central locking solenoid had failed.
Arm your self with a torch a small flat bladed implement and you will need a small torx bit (philips screw driver may do at a push). And a boot solenoid
Put back seats down and get into boot. Unclip the two rear light cluster covers and bulb holder. Use torx bit to undo boot closure strap. Look around the edge of the carpeted cover and you will see some pop in plastic studs, use flat bladed implement to release. Look carefully in the centre as well as there are more there. Once the cover is free, carefully manouvere out of boot, place to one side.
Get back in boot and you will see the locking mechanism. Look closely at it and you will see the rod that connects to the solenoid to the lock release mechanism, use this to unlock the boot.
Now you can get out of the car and work on the boot with it raised (much easier

)
I wasn't sure what the problem was at first, so I had to take everythin apart to see what was going on. I then relocked the boot and waited a few days for the solenoid (ebay)
Hope that helps
Roland