It is a fairly easy job if you have some decent tools and a small two arm gear puller (use it to remove and replace the exhaust valve cap)
The below is the repair kit, can also be purchased from the dealers
http://www.brit-car.co.uk/product.php?xProd=139015
this is a quick guide from the disco 3 site on how to remove the compressor
http://www.disco3.co.uk/forum/topic67721.h...eplacement
use pleanty of penetrating oil on all connectors befor removal