Now this was a big step to help my understanding!
I always thought the viscous part was the link to the front prop, not at the rear prop. Of course it's not looking at the pics (my box is the wrong way round to confuse things).

Does this mean that when the viscous locks, the drive split remains the same, just more 'drive' is sent to the front or does it lock the reduction gear?

Edit:
Answered it myself, no it just transmits the drive but with the same torque split. I need to get a centre diff made to give me 50/50.