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Thanks, both, for your answers. I could imagine it coming from a swan, though there's no lake in the garden. It's been left less than half a mile from the Abbey lake, which is very popular with swans. However, with that long, shallow run-up, I think a swan would have struggled to take off again in the space available. Unfortunately, I have a feeling that, simply by finding it, I may have inadvertently rendered the death penalty, since I only saw it after I'd hauled away the nettles and, no, they hadn't been flattened. I haven't disturbed it since but I have been working within feet of it for the last few days. I've been away from AB for a while, so I'm not sure how to upload a photo here but I'll give it a go as soon as I can. I've had a look at some other images on the t'internet of swan, goose and even heron eggs but none of them looks quite right.
Thanks, again for the answers so far; maybe. if I can include a photo, things will progress.