I'd probably sum it up like this; the Nextbase does everything the Blackvue does and then some (polarising front lens for example) but it doesn't have Cloud capability. You can remotely access the views of the Blackvue if you have WiFi in your car. (I'm talking about my Blackvue, not ALL Blackvue models).
The Nextbase is also a bigger lump and is more obvious in the windscreen.
That's immensely helpful, thank you.
I habitually research every product I buy thoroughly and it's a practise that rarely lets me down, but you have to be careful of the sites that recommend the most expensive simply because hat's the one they'll make the most commission on if you click on their link to buy it. But it means that so far my research has been limited to watching a handful of Youtube dashcam videos and looking at which cameras seem to fair the best.
Nextbase is the one that keeps coming up and seems to be very well received, but it is the size that puts me off a little. I wouldn't want the hassle of having to remove it every time I step out of the car so i don't want it to be too obvious. I don't have WiFi so there would be no point in paying more for that function, but I'll have a look at the Blackvue range anyway to compare. Main thing is clarity, including night vision. Without that it's a waste of time.