usually battery light means alternator is not charging the battery.
i had a battery light on my old 1.9 tdi and it was the pulley on the alternator had gone bad...does yours rattle?
i'm crap with electrics so no help there i'm afraid.
Yeah everything checks out ok so far thus I now need to find out where it takes this reading from.
I tried the air-con trick on no25 for voltage when it first happened and it did show low at 12.1 but when back up to 13.7 and has never dropped since. It does check out with 14.4v with a meter and I done 300 miles yesterday with lights on etc.
Note: The battery light must inhibit the heated seats and rear heated windows to help not drain the battery.