I've just removed and refitted an air compressor on a Disco 3. I read articles on it and watched YouTube videos. Most of the articles mentioned that the most difficult part is removing and refitting the bolt at the top of the compressor mounting as it is very difficult to access. Most people recommend using a 3/8 inch socket with an extension and flexible coupling. I couldn't get this to work at all. I eventually gave up trying and used a 10mm ratchet spanner to remove it. When refitting it every time I offered it up using a spanner or socket the bolt would drop out of the hole. Eventually I had the idea of placing the bolt in the housing and then fitting a small o-ring over the bolt to hold it in place. It then took literally a couple of minutes to fit the bolt.
Hope this helps anyone in the same boat