Geck 540 motor drivers??

[m_c]

Have you tried an RCD in the shed itself?
The RCD in the house will be connected to the house earth, which the shed will also likely be earthed to, but if there is anything wrong with the earth wire to the shed, then the shed itself may not be earthed. Trying an RCD, or one of those testers with the three neon lights that plugs into a 13A socket, will tell you if the earth is actually good.

Looking at your control box wiring, the main power supply to the drives should not be daisy chained. The diagram on the Gecko site at http://www.geckodrive.com/support/st...ly-basics.html shows how they should be connected. If connecting everything to the power supply is not practical, run short wires from the power supply to a star terminal, then run the wires to the drives from there. For the star terminals, a couple suitable mounted/insulated bolts are ideal and just use ring terminals to connect the wires.
Daisy chaining the logic (5V?) supply is probably ok, but I'd personally be inclined to add an electrolytic capacitor over the power terminals of each drive to ensure the drives get as clean a power supply as possible. Do that for both the logic, and the main power supply feeds at each drive.

What actually stops working on the BOBs?
It may give us some idea as to what is causing the problem.