I realise this isn't the most difficult thing in the world to do. But with fitting charges from £30-£120 I decided to save some cash and give this a go myself. I chose the Nextbase 512GW for several reasons:
I wanted the option to record speed
It had to be removable so it didn't get stolen on the first night
It was on sale!
With that said, I also purchased the hard wiring kit. This effectively gives you a piggy back fuse connector for the wiring meaning any idiot can connect it up. And I did! I should really have taken a picture of the wiring kit! Doh! But, you get adapters for the both sizes of fuse in the fuse box so you can effectively piggy back it anywhere it'll fit.
So to work, first I needed a suitable location for the inverter near the fuse box.