They're locally sourced fertilised hens eggs - a variety of breeds (Barnvelder, Buff Orpington, Cream Legbar, Cuckoo Maran). Often we use Thrift Farm near Milton Keynes. This time round, they were short of eggs so I found (via the internet) a couple of other chicken people in nearby villages to source from.
Below is the inside of the incubator, with the lid raised. This was the first chick to emerge (now housed in the hamster cage mentioned above). You can also see (towards the top-left of the picture) another chick pipping its way out. I think the chirping of the early chicks encourages the others in their efforts to break out of their eggs.