I have 94 disco and the passenger door behaving similar.
Opens from inside fine, but sticks with the outside door handle.
Have found that the latch is catching somehow when door closed, openning from outside handle and pulling outwards.
But have noticed that when door closed, openning from outside handle and pushing door inward the latch is able to snap open then can then open.

Have applied liberal amount of WD-40 but still sticks, also noticed that the lock is full of dirty.

Am currently installing generic Central Locking kit so panels off and will get to the bottom of this door lock problem.

Re. generic (non LR) Central Locking kit (for people searching), had problems installing and getting to work reliably when lock motor level, found LR have a locking mechanism that has a rotating part that means motor force not directed in a straight line so motor would open and immediately close again as motor not completely openned because LR mechanism not completely opening. Solution: changed the angle of the motor to point down at about 30 degrees so that the motors straight line force is maximized thru the LR's rotating mechanism, works brilliantly and reliably. Haven't check if this is affecting the windows opening ability yet (keep forgeting, hope not).

 

My father fixed the rear door before, but the front door has a metric lug on it, and all our parts are in american sizes. ( Its weird how they use metric on a british truck) Anyway, I worked out a system, open rear door first stick hand through crack, lift lock pull latch, presto.. I tried open sesamie but that didnt work =/