Hi all, thanks for your replies and thanks
@Tell for such a detailed reply too!
I was concerned the issue was because I formatted the cards, so I ended up buying a MIB2 V6 card from eBay:
Link. Also, as DaveS suggested, I bought a new SD card reader to I could perform the whole operation on an entirely different PC.
I didn't format it this time of course, and followed exactly what Daz3d describes, and still I'm stuck on "checking navigation data". I also tried the same card on my father's 2015 Leon with MIB1, and it does the exact same thing. So either it doesn't work for all cars or I'm still doing something wrong.
Interestingly, if I try my fathers original MIB1 card in my car it says the navigation data is not valid, why is that? It works fine in his car.
Does it definitely work with all MIB2 cards or is there some specific card I need to buy?