Personally, I would go with fibre and media converters ... Might not be as expensive as you may think.
Followed by
An interesting thought, thanks, mixture. I fleetingly thought of fibre and then discounted a nano-second later precisely on the grounds of cost-benefit. I'll take a proper look so I'm prepared for my Sunday afternoon conversation with network designer friend.
I had the conversation with said friend yesterday and although he stressed "I wouldn't normally advocate running copper outside a building and fibre would certainly be the norm" we agreed that in my case the cable can be run in an existing cable conduit and looking at the topography involved the risks are low. Cable it will therefore be, and I have my next project
Cheers!
XV