I had APT differential guards on my Discovery 2. I was very impressed by the build quality and fit of those guards and so it not a hard decisions to fit them to the Defender.  

I bought my differential guards from Peter and Nick at PCB Land Rover in Adelaide. I like PCB Land Rover as everyone there is very knowledgeable about the vehicles and are extremely friendly and helpful. I think they are the distributor for APT products within South Australia.

The rear guard is very similar to the one that went onto the Discovery 2.  The front one was very different design but very easy to fit and a fair bit less fiddly that the rear.  

Below are some before and after photos and a video of Ben Woodgate from APT Off Road fitting a similar guard to a Defender