When I did my Disco 300Tdi, procedure was totally remove engine mounts, and lower engine pretty well as far as it can go. Then the lad that was helping me put two 12" extensions on a bar, and fed it along the top of the box from under the car. I reached in from the top, and when the engine is dropped like this, I was able to make sure the socket was snug on the bolts. As far as I can remember, they were all the same size head. I would imagine that this is bloody difficult to do on your own, but if you can get someone to hold the socket in place like this, it will be much easier.
Good Luck!