"Handbook of Avian Hybrids of the World" (Eugene McCarthy; 2006) records hybrids between black swans and the following:
• mute swan
• whooper swan
• tundra swan
• greylag goose
• Canada goose
I've never seen a swan x goose hybrid. Where is this National Trust garden, please? I'd like to visit to see the "swoose".
West Green House Garden. Hartley Wintney. National Trust. If not member £8 entry. But as I said, you cannot rely on seeing it now as it often flies out with the Canada geese during the daytime and their opening hours, and it wasn't there when I went, the swan mother being alone on the lake.