Tested, and all readings were above 12.5.
Took my starter in to a small electric shop, pulled the top off, and about a pound of dust and dirt came out and the brushes were frozen. Heavily worn. Pulled the switch off the top of the starter and the electrical contact point was also very worn. The guy was quite surprised it had been working up until last week. Needless to say, it's getting a full rebuild.
