but this is
the brightest one I've seen, and might even be a foreground
object. Your time-series might be the deciding information!
I reckon your right - that its not in Andromeda but is rather an SN on the edge of our own Milky Way - could be wrong though! I was shooting M31 on the night in question - 27th and captured this also (though not as clear as Dave).
Sorry Dave can't be of assistance, though it looks like you've got it very clearly.
  