Thanks. I was going to reply with an image of the error but Mobrockers beat me to it.
The PARTUUID line in my /boot/cmdline.txt fixes the error.
For anyone wondering, my /boot/cmdline.txt now looks like this:
root=PARTUUID=000f07e3-01 rootfstype=ext4 rootwait quiet osmcdev=rbp2
Got the PARTUUID like this:
osmc@osmc:~$ blkid /dev/sda1
/dev/sda1: UUID="5d31425a-288e-4963-be07-534286b8ec2e" TYPE="ext4" PARTUUID="000f07e3-01"
Think I was confused as I seem to remember something about UUID support being removed in Raspbmc? Anyway, thanks all.