If anyone's interested, I did get the Ctec battery monitor, connected it directly to the 12v battery & placed it in the little box set into the battery cover under the rear seat, it fitted very well. To check the battery voltage, just pop the small cover off & check the led's, this also gives a convenient plug to charge the battery from a Ctek smart charger.
However, it seems the 12v battery is hardly ever fully charged & the monitor is usually showing red, so no real help.
I have had the LBX for 2 months now, being retired, we don't use the car a lot & had no problem from the 12v battery & I've stopped worrying about it.
I have, however bought a small battery booster, just in case.