Thx all. Took the battery out and it started charging. Must not have been getting a proper connection in car.
I just have to keep an eye on the car now to see if it discharges the spare battery. I guess if it does then the fault is on the car.
Car would not start so I hooked the battery charger up to it and it is showing between 5 and 6V.
Battery is less than 6 months old and no problems until now.
Am I right assuming this is the battery and not the car? Even a drain that left the battery flat should show more than that and be able to recharge - car has not been left stood for more than a few days.
So before I cart it back to the parts store and make an idiot of myself, faulty battery?