Ironically, the day after I said we had no problem with egg eating - both the chicken eggs went today. They were laying this morning when I collected the rest of the eggs and I didn't get around to going back until late afternoon. One had been smashed, shell mostly gone and yolk all over the nest box. The other one had a big hole in the side, had been moved to a feeding shelter and emptied. No idea who did it. Might try the mustard but meantime have put fake eggs in the boxes.
H