Have managed to unlock door! Had a look at a BMW document on Central Body Electrics zke111. This has a view of the internals of a door lock actuator. Shows a locking rod which is offset in double lock mode. Was able to get a hooked wire inside the actuator from side where Bowden cable attaches and disengage this locking rod, then pulling bowden cable (internal handle) released lock! Finally!
Link to pdf:
http://www.bmwmotorsports.org/BMW_docs/x5zkeIII.pdf
Photos on p24.