Wait and see. Looks a bit like a wallflower to me.

Wallflower was my initial thought but further examination put doubt in my mind. But I don't know what it is. Oenothera macrocarpa maybe?????

Yes, now that you say it, I think it is Wallflower. I remember it from last year. Thank you.

I've looked again. Sure enough there are some wallflower flower buds visible at 11 o'clock near the centre. Should be in bloom soon.