Right so did it all! It's fully working, no error codes.
Wanted to share some tips / tricks and what went wrong so it might be useful to someone else. It goes without saying that this is a risky process - only start if you are comfortable with what you are doing, and can fix it (or afford to pay someone to fix it) if you break it. It's also worth noting, I tried to do this myself to save the ~£160 cost of unlocking it at a dealer, but I ended up having to pay £90 for ODBeleven in order to complete the job, so didn't really save that much money in the end.
Note that my car is a MY2016 with MIB2 Amundsen. Remember, your car might be different.
Step 1 - Research
The first thing I did was read as much as I could, watched as many videos as I could, to make sure I knew as much as possible what I was doing before I started fiddling with anything.
Step 2 - Update firmware
Before doing anything else I wanted to update to the latest firmware for my unit - in this case I was running 0241 but the latest is 0253. Thanks to @varooom I got the latest firmware for my unit. I simply put this on an SD card, put it into the unit in the glovebox in place of the nav card, and ran the firmware update process on the MIB (press & hold MENU, then navigate the options - pretty straightforward).
Step 3 - Unlock "Green Engineering Menu" / GEM
I have a basic ELM327 OBDII-bluetooth adapter, and Carista - so I unlocked the green developer menu using that. In the Carista options, it said it was for "Discover Pro" only (which I think is VW's equivalent of the higher end Columbus) but it worked anyway. This video might help you understand how to do it, although it's based on OBDeleven, not Carista:
Step 4 - Load the "MIB toolbox"
This is pretty much the same as updating the firmware, but instead you need to load on this software https://github.com/olli991/mib-std2-pq-zr-toolbox/releases/tag/v1.2.1 . I encourage you to read the readme on https://github.com/olli991/mib-std2-pq-zr-toolbox - for my version of the firmware (02xx) it was necessary to copy some files around, before putting them on the SD card and doing the firmware update process. Check out this video:
Step 5 - Unlock Smart Link
Next, use the toolbox software to unlock Smart Link. Here's a video that explains it better than I could:
Step 6 - Dispare
At this point, from everything I read it was supposed to work. I had Smart Link in the menu, and it no longer told me to contact a dealer - but I couldn't get it to connect to my phone, no matter what I tried. A bit confused I had to do a lot of research. It turns out I had 2 new problems I needed to fix:
1) Turns out there is an extra step to code the stereo to activate Android Auto & Apple Carplay
2) An error code - now the car is running updated firmware, it throws an error code as the version doesn't match what it thinks it should be running
I was trying to get away doing this without OBDeleven or VCDS, but turns out these last 2 steps are impossible without one of those tools. So I ended up buying the OBDeleven Pro Pack for circa £90 from their website (it's 30 cheaper than Amazon).
Step 7 - Activate Android Auto & Apple Carplay
This video explains what you have to do in VCDS or OBDeleven to enable these options:
Step 8 - Remove the fault code logged in the stereo
There was a permanent fault code logged in the stereo as the stereo was running a newer firmware and something didn't like it. Another video to help fix this:
Step 9 - Android Auto
And voila! It works!
Hope this is helpful. I wouldn't have been able to do this without firmware from @varooom, the fantastic MIB toolbox or the excellent videos by mr-fix on Youtube.