No need, I can answer this for you. Anchor worm will persist in the pond - they won't disappear completely just because of a mild English winter. The pond would have to be treated with an anti-parasitic or completely dried out for several months. Rana temporaria shouldn't really have issues with anchor worm so I wouldn't blame the anchor worms for that. Did you experience a cold snap (freezing?) recently? That can kill the occasional common frog.