So I've kind of worked out the fill/empty rate now. 2 weeks of drought, 3 for sure, will more or less empty it. 2-3 days of steady rain will fill it again. So, I'm theorising, it'll spend most of the summer emptying and filling, and most of the winter, pretty full.
It's quite impressive to me now, though. How many 100s of gallons of water is it holding now? Several, anyway. All of which would be waterlogging the beds which I'd therefore be unable to dig over and plant out and cultivate. So it's doing its job and it was worth all the hard labour, mud, and cursing of the winter months.
No frogspawn. Yet. But I saw the 1st pond skater, Gerris lacustris, yesterday. The whole surface seemed to be busy with something I couldn't quite see. And bubbles appeared, though I didn't get the impression they were floating up from the depths.