So after I'd voted I went out to the pond in my sister's garden to get some weed for experiments for work. They warned me that there were quite a few tadpoles, also frog and toad spawn, so I ought to be careful not to take any. And indeed the water was churning from all the tadpoles eating goldfish food. Not to mention the fourteen adult frogs I counted clustered in the water, mostly around the edges of the pond, presumably the survivors from last year.
Not bad for central London and a pool that's only a little larger than a bath. At the weekend I'll try to get a few photos if they're still there.