The ones I set after they lay. I just collect and pop them in. Always seems like the ones I want to hatch never lay until the incubator is set with eggs, then they all start to lay
If I'm collecting eggs from a group that I get sporadically, I save them for up to tens the set them. If you notice a broody hen can build a nest and lay eggs for many days until she sets fully same concept.
Welcome to BYC Matt I hope you have a great hatch. That is a lot of money for Mottled d'Uccles. I sell mine here in the states for 30$ for 8 and that includes shipping