Thanks for the suggestions; Dermestidae may be the answer. When you get a handful of wheat and look at it closely, you see it has moved - they are absolutely tiny - the size of dust. Can't see any beetles at all - just moving "dust" !
It was very dusty wheat in the first place - more like seconds, and our hens have not been laying so well this year, so I'm blaming the wheat being low in nutrients.
We're dumping it outside in a gully, in the hope that it will tempt the Rock Doves away from the chick rearing pen, the ducks and the hens ! The muscovies and turkeys are still getting it - until the new wheat comes in.