I've just candled the egg to see if it was alive because I though it was dead, and everything looks good :)
Judy is sitting on it fine, keeping it warm.
When I candled it the air sac looks a bit big, but that should be ok. The chick was moving inside the shell!
I still think its amazing that a chick can develop out of something as simple as an egg, and then know what to do to get out and survive.