Chongo,
Back door lock access on the DII is much more difficult compared to Disco I because the lock itself is not readily accessible from the outside.
But try this simple fix. Park the Disco II with the nose down, e.g. on a steep driveway. Use the remote release to unlock all the doors (back door may still not open).
Using the palm of your hand, give the licence plate light holder (where the LR logo is) several hard and swift whacks.
Try the door now. It should open.
As for the Pre 1999 Disco Series I, the lock mechanism can be accessed simply by prising the Land Rover green oval logo off the licence plate light holder. You should then be able to jimmy the lock open.
Lawrance Lee
'99 DII V8 auto
Melbourne, Australia