Thanks for your ideas. Sadly Liz thought some had survived as no more were floating, but it was because they were dead on the bottom. I checked with her and she does have a filter, so we both reckon it must have been the rain. At one point it was coming down so hard it hurt to be out in it, perhaps they were literally beaten to death by it. :'( :'(
Anyway, she's decided not to bother with any more and just leave it to the frogs and newts. :'(