My battery sits where the jack/chock usually sits, I removed the airbox as it use to come in where the battery sits (ABS spec) so I put an inline one in:
Ian, I would look at making a battery box in the rear up for your 2nd battery, are you at tyes green sunday?
The relay I use is this one :
You can see it wrapped up in old tyre inner tube for protection just above the battery, well that one is actually my winch relay but its the same one, my split charge one is on the other side by the original battery:
You can see all my accessory wiring there as well, lights, radio, cb e.t.c all run off second battery. The wiring is so simple to do as per the previous diagram, the only difference I have made is I use and ignition live and not an engine live, this means with the ignition on but engine off both batteries are connected, the idea behind this is if I get a flat starting battery I can back charge just by switching my ignition on, after a few mins enough power would have transfered across to start the engine. If you run off the ignition light curcuit from the alternator then only when the engines running will it connect the batteries.
The relays are about £14 each, I can get a roll of thick wire if needed although my large cable crimps are broke, I can still make wire up though using soldered connections if needs be but its a make it up at installation jobby really as you wont know lengths e.t.c.
I have also now installed an electric fan to help with any future overheating issues:
