Go slow, avoid wedging a screwdriver in the seam, make sure you have all bolts and screws removed.
I use a length of wood dowel and a light hammer and tap gently around the circumference and LIGHTLY on the crank until the joint splits. The 850 has roller bearings, so this will not impede the separation.
I personally don't recommend the pullers that Old Britts sell as I am concerned that too much pressure can warp the cases or strip the threads for the primary case.
The one I'm referring to is the manual published by Norton Motorcycles (a subsidiary of NVT Ltd) back in the day. It's been reprinted and new copies are available from any good Norton parts dealer. Definitely worth having in your library.