Despite having decided I wouldn't get enough use out of it to warrant buying a GoPro I just ordered a Hero3+ Black edition from Currys' reserve and collect service. Same price as Amazon - £269. I just wish I'd had it for Silverstone.
I went for the 3+ because of its ability to automatically slow the frame rate to preserve clarity in low-light conditions, which seemed quite a smart idea. The 4k feature is no use to me because I neither have nor desire a 4k TV. Also, it shoots 4k only at 12fps so it's of questionable use anyway. The Hero4 will apparently shoot 4k at a decent frame-rate and seems to be set for launch around October. Would it have been worth waiting? Dunno. But, you can wait forever and there'll still be an upgrade just round the corner. As I'm generally of a replacement rather than upgrade temperament it probably won't trouble me too much.
Before settling for the GoPro I was tempted by the Garmin, because it has some quite interesting GPS data overlays built-in - speed, G-force, altitude and so on. The GoPro won because it comes with everything I need whereas the Garmin package excludes a remote control (£40 extra) and decent mounts. All in all the GoPro seemed pretty good value (relatively) and should tolerate most abuse.
I look forward to buggering about with it at Brunti on Saturday.