Turns out I'm a bit of a dork and a spot of googling could have helped: The line I needed in my config file is:
vo=x11,fbdev2

Putting "vo=fbdev2,x11" doesn't work somehow, as if the fbdev2 succeeds from inside X11 but doesn't display anything.

Yes, fbdev2 will succeed from within X11 - if you switch out to the terminal, you'll see the video playing.

As an aside, when specifying vo in your config, it's often a good idea to end with a comma eg
vo=vdpau,xv,fbdev2,
The final comma will make mplayer use the vo specified in the default config if yours all fail.