This morning when I checked on the chickens the other 3 unhatched eggs had been tossed out of the nest - I have noticed that the mother's 'chest', which has no feathers over the incubation stage, is not so hot now. The chickens are doing fine and their wings feathers are starting to show and mum clucks around giving them food. The weather is not very warm so I don't think that they have been outside either.
Fortunately there has been no sign of the rat, and I have laid another trap outside and under some wood (quite an ingenious trap too, I thought!).
Another few more days before I can expect some action from the other clucky mums and their 'foreign' eggs - these are Blue Andulsian which I bought from TradeMe and are supposed to be fertile.