My bet is still on a local set of thieves (even if they are then supplying into a larger thieving/"fencing" network)!
A question though [member=27141]DavidandCollette[/member] - any knowledge of other agri-theft in your area at about the same time (from small-holders or larger farms)? None perhaps ??!!
There are obviously roving gangs of agri-thieves around the UK looking to take equipment or live-stock, but, to my mind, it wouldn't be one of them that went to all that trouble to take your old Kubota.
Frankly, DandC, I reckon you need to keep an eye on who comes and goes on your bit of land.
A personal experience: I frequently keep expensive gardening machinery in my Estate car which is parked on the street outside my house. Then, recently, my large garden shed was the only one in the neighbourhood that was broken into one night! The toe-rag/s didn't know though that I don't store any kit in the garden shed anymore - tough luck you little sh*t/s! However, I have a very mangled shed door (which required a lot of effort to break open!) and the only things they found and decided to take were a few cheap hi-viz life jackets - I guess they fealt they had to get away with something !!!
Think local and beware I reckon.