New alternator and battery, still low voltage.

[jamesgmg]

So that means your battery light will be working fine but you will still get the low voltage when its cold which is correct

The battery light supplies 12v to the exciter on the alternator. No battery light, meant no exciter, so the alternator was doing nothing.

Once I'd sorted the fuse, the alternator is kicking out 14.4v to the battery immediately at cold idle, so perfect!

I didn't know alternators had "exciters" until today!