Thank you for your advice - that sounds much more humane than other ideas people have suggested.
I live in a very urban area - but have seen a heron - 3 years ago and a hawk - last year.
But we do have plenty of urban foxes so I layed the blame there.
As for cats, I have a cat,who is not interested in the fish at all but is a very good gate keeper. God help the cat that comes into our garden!
I also have a dog who is adverse to all cats except ours. So, as I say, I think the fox is the most likely.