The choice of distro was taken out of my hands. CuteFish wouldn't boot when installed – even though it ran fine from a USB stick. Elementary OS installed but suffered from the issue many flavours of Linux do on older Apple hardware: an issue with Broadcom drivers means WiFi doesn't work. Despite following a few different guides I couldn't work around it.
Next up was Zorin OS Core. Somewhat surprisingly, this installed, booted and had working drivers. So this is what I'm sticking with for now. If it works out I may upgrade to the Pro version with different desktop layouts.
I may loop back to CuteFish in the future, once it has gone through a few more iterations, as that is the one I really wanted to try but Zorin was a close second. It's just a shame you can't get the MacOS look without going Pro.