I've noticed that in an "emergency stop" the rear wheels,in fact the rear n/s particularly on our 2006 Defender 90 seem to lock up first, meaning the rear end steps out (wet or dry roads). Cars have a compensator on the rear brakes to reduce the brake force as the rear wheels are unloaded in this situation. Am I to assume that this is an aspect of "4x4 control" to be aware of on our (non-ABS) Defender? Or is there something that can be adjusted to alter the rear brake force relative to the front? Normal braking seems fine and when tested it pulled up straight with hands off the wheel.

I would expect the rear to lock up first on most non abs cars, particularly cars with a high center of gravity. The rear wheels don't do too much stopping as most of the force is borne by the front wheels as this is where momentum is trying to push you. You should try cadence braking. When you perform an emergency stop, when you just start to skid/change direction release the brake a little to allow the wheels to rotate a bit, then hard on, then off, then hard on... until you no longer need to stop, or you you have, er, stopped.
Its a good technique, though hard to practice as its not often that you will need to perform an emergency stop. If you can find a bit of road (pref private land) to practice then it could save you a lot of cost/pain when you have to use it for real.