Right, I'm incredibly pissed off, upset, sad...
If you haven't linked 1 and 2 together already then here's a piccy.
Anyone heard of those handbrake buttons somehow releasing themselves or not latching properly all the time? I've never had a problem with mine before.
I definitely put the handbrake on because the car would've rolled away as soon as I let off the brake so I can only imagine the button somehow failed. It didn't even happen straight away while I was walking off which is weird, it was about 5 mins after leaving the car as well so I'm stumped. Lesson learned here, always park in gear and point wheels towards the kurb on hills.