Have you tried this approach:

http://www.bmwe34.net/e34main/trouble/Locked.htm

They can unless they are broken -- ie there is a plastic part inside that
can break, and doesn't effect the power actuation of the lock, but confounds
you when you need a manual activation.  So try the method linked at
http://www.bmwe34.net/e34main/trouble/Locked.htm  by bfd, and if it doesn't
work, try the passenger door.

Finally, there might be a manual release for the hood (bonnet) from outside,
acccessed by sliding a very long straight screwdriver in through the grille
and rotating the release point, not sure where it is on the E34 exactly but
I'm pretty sure there is one.  Look carefully with a flaslight.  This is
intended for use when the hood release cable or handle breaks.

Dumb question, is your battery under the hood?  Or under the seat or in the
trunk (boot)?

-Russ.

If you can't unlock the doors mechanically by using the key, you have
problems not related to the battery. The doors, the drive door and the
passenger door but not the rear doors, are mechanacially connected to the
key locks. The trunk has a manual opening position where the key
mechanically opens the locking mechanism.

In my car, the battery is in the trunk, that is why I bring this up.