Well I had 12 eggs in the incubator - 7 RIR's and 5 Buff Orpies.
5 days ago, one of the adult Orpies went broody so I say her on some pot eggs next to the incubator. When the eggs started pipping I put 6 eggs under the hen, and left 6 in the incubator. As the incubator chicks hatched I sneaked them under the hen and she's been great! The eggs that didn't hatch weren't developed fully, although fertile.
Now I don't have to worry about heat lamps etc - I can leave the chicks to their new mummy!
10 lovely chicks all happy with their new mum!