Thanks for the info. I had suspected that the hatchery had sold me a dodgy batch of eggs. I bought 46 Plymouth Rocks and 10 Leghorns, of which 12 eggs hatched. And now one of the 2 Leghorns is a cross!! I will keep the cross and see how he(?) develops.
Theoretically I have 10 Plymouth Rock chics and 2 white Leghorns, all living together. They are about 3 weeks old and one of the Leghorns is predominantly white with what looks like Plymouth Rock feathers. Is this colour pattern possible for a Leghorn? I bought the eggs from the internet so...