Anyone keep hens?

I'm not sure what 'Subject' this should come under, so apologise for straying into the pets domain.

I have 4 x ex-battery hens running free range, and all have been laying well...BUT over the last couple of days one of them has been getting to the eggs and eating them.

I know this is a fairly common problem and I've tried laying a stone egg in the nesting box, filling an empty egg with mustard but to no avail and am getting lost for ideas. As yet I can't identify who's doing it, and I will not cull 'the culprit'.

Have any of you poultry keepers come up against the same problem and managed to stop it?