Something clearly went wrong here....
Yesterday I went to the nest box to collect the eggs and very nearly missed this miniature egg. It's definitely from the chickens as it's shell matches their normal eggs, but it's tiny, about the size of a cadbury mini egg. Normally they lay creme egg sized eggs (on the top left), which given my bantams are half the size of an ordinary hen is still pretty impressive, there's a normal egg on the right for scale.
Who knows what happened, the egg itself is normal, with a yolk, it's just like it's been shrunk however. The chickens are there usual perky selves, with the exception of the one who has gone broody already, so I don't think it's a problem, just a rather intriguing bit of chicken biology!