Its an battery pack (piggybacks onto the camera to extend battery life) rather than a 2nd battery, they are normally £40 I assume this comes with the bigger back door for the waterproof case etc

Here – http://www.argos.co.uk/static/Product/partNumber/4178165.htm

Question is, this GoPro deal or an SJCam 4000+ for £92?

Was just about to post that offer.

Sorry Drac 8)
Ive reserved one to collect in case, but still unsure as I think the SJCam is a better package if the video quality is comparable

Based in my SJ4000 (not 4000+) I’d go with the GoPro at that price. I’d say the video quality is as near as makes no difference the same, but the build quality, on my SJ4000 at least, is pretty poor. For example, getting the buttons to recognise being pressed requires 10,000 tonnes of force which makes it very irritating to use.