Thanks for your replies,
The food is in a feeder, but they flick it out. Maybe I shouldn't mix layers pellets and corn in the same feeder? (they seem to flick it all out without showing any obvious preference for one or the other)
Womble: could you describe your wooden lip, please? I'm not sure what it should look like.
As an aside, the problem's been inadvertently solved, albeit temporarily - I opened up the hen enclosure to allow them out into the sheep paddock - and the sheep came in and hoovered up the scattered hen feed.
mab