Hi all,

I've had a nose through the search but I've probably missed what I was looking for. My lad's 110 was originally military police and had a dual charging circuit (so SVO tell us), the guy he bought it from has done an engine change at some point in it's life as well. My son would like to put it back to dual charging and put a second battery in it (along with some electric work I want to do - some donkey has wired everything with red wire ), my question is - how can I tell easily if it is still dual charging, is there something specific I need to be looking for?

Take off the positive on the aux battery whilst the engine is off, and check the lead is not still at 12v.

If its 0v without the engine running, start up and hope it goes to 14v

As JulianF says, but if it has a smart charger for the auxiliary battery then the disconnected positive lead won't have any volts on it because the system knows the battery is absent.
If you are installing a split charge system you may want to consider a VSR (voltage sensitive relay).

Easiest way is to use a relay intended for towbars- these will activate the 12v output when the alternator is charging the battery. Wire the input side to the starting battery and the output to the auxiliary. When the first battery is charging, the two will be linked together. Turn off the engine and the batteries are separated.