Flavia, I don't know exactly what it is but it wouldn't worry me if it were mine. I had my courgettes outside hardening off during storm Hannah. They're bruised and a bit mashed but I'm confident they'll be fine once planted out. In London you should be able to harden yours off now and plant them in a week or two, especially if you have fleece, cloches or some other protection.

is it just that one leaf??

can you rule out it being sprayed with anything you might spray in the bathroom ... ???

its green ... itsstill doing its job ... when the babe gets a few more leaves .. and might simply fix the issue.. by snipping off that leaf.. if its the only one in the bunch ... one might say.. out of sight.. out of mind ...

ken

I wouldn't even bother to snip it off. Once it's planted out that one leaf will be neither here nor there.