There is a chap on the seriously series forum who has developed a kit that uses standard defender calipers and disks and just attained European TUV approval. They were mentioned in "The Landy" rag
http://seriouslyseries.createaforum.com/new-board-3/disc-brake-conversion-diy-style/75/
https://m.facebook.com/yakyakclassics/posts/1530903977164742
Teh only downsides at the moment is that he is now looking into manufacturing and also that due to clearances you cannot run standard LR steel rims.
The advantages are that they are standard LR parts.
As for installation though you have to look into the volumes of brake fluid your brake mater cylinders can move. He recommends fitting a Defender Master and Servo and then of course with this you will probably need to fit a restrictor to the rear axle to stop them locking up.
Not to hard in a 2/3 but a lot more to think about on an S1