This morning I found something interesting in the nest. My first thought was that I shouldn't have been feeding the chickens their egg shells as they now seem to have developed a taste for eggs, but on closer inspection I could see that the shell was very thin, so instead of an egg eating monster I have at least one chicken (Cissy by the looks of things) that's not getting enough calcium to produce strong shells.
Now that I think about it, they seem to have been pecking the grit pot a lot... I've topped it up with grit mixed with (baked and crushed) eggshells, and they're not getting any treats for a couple of days. If that doesn't help then it's time for more drastic action, but I'm keeping my fingers crossed.