Good news - we now have a third resident who turned up late on the 23rd and went straight into Box 1.
It then roosted for the next 2 nights and is accompanying our pair from Box 3 on their feeding flights and returning to Box 1 in between. It did occur to me that maybe it’s Box 1’s partner turned very late, only to find that his ‘wife’ has now shacked up with the neighbour in Box 3 and is sitting on two eggs. Oops! Never mind, we’ve seen a party of five flying about the house - we think three of them are ‘ours’ and the other two just hangers on so hopefully the single in Box 1 will partner up with one of them. We shall see! Meanwhile the pair in Box 3 are now taking turns in incubating so this makes hatching expected around the 15th June