I have been doing some research and according to the bluetit blog on the BTO website, the bluetit lays just one egg a day. She will start incubating just before she lays the final one (how does she know it is the final one??!!) and then it will be about two weeks incubation. They lay anywhere between 5 and 14 eggs! From that, I presume she will have finished laying when I see her decide to stay in the nestbox during the day rather than just at night. Not sure if the female does all the incubation though.