This is probably a stupid question but what if you leave a spare battery in the boot connected to the cig lighter (aux) socket in the boot which is permanently live? Will this effectively put the two batteries in parallel thus increasing amp hours? Then both batteries would be charged whilst driving. Would there be problems with the vehicle electrics not going to sleep over night? Or other really nasty things?

Just a thought. 2012 5.0L S/C
Previous:
3.6 TDV8
4.4 HSE
P38 4.6 HSE
Classic 3.9
90 County
Series3 Lightweight

Nothing to stop you doing that, the battery would have it's charge state maintained. You effectively increase the AH by the capacity of the 2nd battery.
I'm not quite sure what would happen if you tried to start the car with the second battery connected to the aux socket, you may blow the 20A fuse on the aux socket circuit.
But you may not.

I think the idea above of the spare battery in the boot has been covered by dan here
http://www.fullfatrr.com/forum/topic5721.html?highlight=battery FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Thanks for the replies. As usual Dan has indeed covered the topic in depth.

It does look like upgrades and additions are required to do it properly so I think I'll stick to carrying jump leads for now.  2012 5.0L S/C
Previous:
3.6 TDV8
4.4 HSE
P38 4.6 HSE
Classic 3.9
90 County
Series3 Lightweight

Dual batteries is slightly more complex - I have them on my Jeep, but a matching pair. It's ok to connect them in parallel when charging, but apparently they drain each other if left like that when not charging. I have a simple relay connected to the ignition and it works just fine.

If the batteries are materially different, I'm not sure if you can just connect them up, or need a more sophisticated split charge system with diodes.