Had a look over the car last night with Nick G, turns out there's a 1V drop between the battery and ignition, so when it's reading 13V it's about right.
Duostyling AFRM was giving it the hump a bit though, it's happier without so I've canned it. Now I need to find another way to mount one neatly, as I've taken up the din slot and cubby hole already.