So I had a look and if you slip out the SIM tray you can see a little tab just inside which you can push out and then slide a spudger inside the back cover and get it off.
But the problem is the back cover has the flash, the NFC antenna and the wireless charging.
And I can't move them onto another back cover as they are glued on so may be damaged.
Anyone have any suggestions? If I could find a full complete back it would be simple as a quick swap.