Thank you for your help, John.
No, we haven't experienced anything other than brief excursions once or twice a degree or so below freezing, and there has never been any ice on the pond. Perhaps the two frogs died of natural causes - old age? Like me!
We can see a large number of tiny tadpoles all over the bottom of the pond, so at least some are surviving.
Perhaps I'll look into anti-parasitics, just in case.
Thanks again,
John