Its only a suggestion Malc, but I would avoid putting the ECU in the battery box, Only because its a dark, damp, lonely place, containing a plastic box full of acid. Rover always kept it in the passenger compartment (RR under seat, Disco by your toes) and I'm sure they have their reasons for this.
I managed to pull enough loom through the big hole I drilled by the fusebox, and running the loom along the edge of the tunnel and across the seatbox, to mount the ecu on the bulkhead between the seats.
Nice and warm and dry, and out of harms way.
I keep meaning to put up some pictures of mine but I'm away from home at the minute and have no pics in the laptop.
Good luck, Dave.