I need to dismantle a Shimano rear disc brake hose to thread it through the closed type guides. Is there a 'better' end to disconnect – lever or caliper end, and do you need to replace the olives or ferrules or whatever gubbins holds it all together?

Lever end is best. You should be able to pull the hose out with a careful squeeze of the lever at the same time.
If the olive fits through the guides you can just leave it on the hose and reuse it. Shouldn't spill too much (if any) fluid either if you're careful.
Pass it though the guides, refit it and test for sponginess to see if bleeding required.
CRC do the olives if you need to cut it off to get the hose through the frame guides.
http://www.chainreactioncycles.com/Models.aspx?ModelID=1741 (Not sure why you now have to buy a new shroud nut)