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I thank you, King Normie and wildwood, and I think you are both right - I think these birds aren't nearly as communally-inclined as the literature would have you believe. The existing and long-standing nests (before we came to live here 4 years ago) are about 25 feet from where we put up the new ones, as the sparrow flies, so I think we must move the dodgy nestbox round to the North of the house and see what occurs.
just a footnote - we have a deck outside the kitchen French doors, and there are various birdtables, a birdbath etc., and most Cornish Summer evenings we will be having a glass or two out there, and these dozy fledglings will peck around our feet, even ignoring our ancient sleeping Siamese a couple of feet away. I say dozy because they have yet to learn their world is dangerous. But I always feel a bit priviledged when a daft spuggie patters over my feet.
Thanks again for your interest