Ian,
Found a few images of when I stripped the rear hub out of a 600d which is pretty much the same as the 850.
The 4 x hub nuts as mentioned will need an air rattler or impact gun. At the rear you will see cage bolts, these will inevitably spin, what I found good was to drive a big flat head screw driver between the top of the bolt and the wishbone frame if you follow me.
To remove the hub you will have to have the drive shafts off, remove the split pin, find a way of locking the drum (I found a long bolt popped into one of the wheel stud holes and tightened as far back as the back plate worked quite well), with the drum locked proceed to loosen the couple locking nut, keep this along with the positioning washer safe and then with a wooden mallet and a lot of penetrating oil give the hub a few glancing and spirited blows. Failing that a regular hammer and a piece of 2x2 timber. No direct metal on metal as you will mushroom the threaded end of the stub and don't use heat as you will destroy the flexi coupler and rear bearing seal. While you are in there do the bearings if you can, they're cheap enough to get and easy enough to do once you can get the old races out of the hub.
If you are stuck in anyway hit me up with questions and I'll do my best to help!