Turns out it is very easy with a Mac, using adb and fastboot. The device came from Gearbest with vendor ROM, so although it needed unlocking it already had the unlock permission. Took me ages to figure that out.
http://www.androidtipsandhacks.com/root/fastboot-mac-linux-recovery/
Hi. Totally new to Xiaomi. Bought my MiMax from GearBest running 7.4.16.0. I understand that MUI 8 is rolling out over OTA. Will I get it or does the fact I probably have a vendor ROM mean I'm going to have to flash it?