AHA!
I thought I'd done that, but (instinctively?), I put a space after the comma.
Without the space (Highlighting your example and middle clicking in an xterm), it works fine.
That's one to remember.
The man page isn't very clear. I thought it might be my mplayer, it's horribly compiler-dependent Every new build with a different compiler fixed something and broke something else. Mencoder is horribly buggy,here.
Thanks.