Thanks for the advice folks. Tala Orchard - I'd not seen a contraption like that before, so it got my interest. I work with solids handling equipment (powder conveyors etc) for a living, so I was trying to figure out how it worked.
Anyhow, I've still no idea, but what I did discover is that if I drill a 19mm hole in the bottom of a blue keg, and put a long bolt through, with another bolt welded across the inside to make a t-shape, a little spurt of grain comes out when the dangly bolt gets knocked, but then stops due to the grain "bridging", just like the chookamatic in your link!
So, I've set the keg up outside as an experiment. So far none of the hens have actually pecked at the bolt yet - they just keep walking around underneath and nudging it by accident. However, it will be interesting to see if they do catch on
.