I decided to buy one from Tesco using my Tesco vouchers and ended up paying £140. I've been out a couple of evenings looking for aurora but haven't sat down with the manual to find out all the various modes so I haven't tried it out yet.
I've also decided to go for a case with battery port access. It won't be waterproof but will keep any sudden shower of rain off and allow me to record for more than the 55 minutes the internal battery allows using one of the battery packs for charging a phone. I seem to recall that the GoPro4 gets warm in use so it might stop the lens misting up at night.
So, nothing to report yet but I think it could be great to leave it recording the night sky at one location while going elsewhere with another camera.