Ian, I've regularly had spotties in boxes here and never known them return. I think open nesters tend to get out as soon as possible whereas hole nesters particularly house martins tend to return for a couple of nights. Whether your spotties will try to nest again is another matter though when mine do its in another place.
Sorry about Sunday but didn't think it would be at all pleasant. I will be leading a walk from ABERPORTH TO MWNT along the coast path on SUNDAY 17th.
Details to follow.