It's a type of rocket ... not shure of the species.

Thanks Hywel, they are standing about twenty inches high, at the moment and they've grown from seed, I just forgot to label them, old age I'm afraid. I'll try looking up rocket to see what turns up.

Alternatively they look very much like Mizuna greens too, once it has gone up to seed. Is that possible?

I think the 4 petals on the flowers make this Rocket (Barbarea vulgaris). The flowers are edible as are the leaves, but some believe the leaves cause may kidney damage.

I sincerely hope not - I love rocket.

Thank you all for being so helpful, I've come to the conclusion it's wild rocket, though I can't say I've found the taste very nice. Next time I'll try and remember to label the seeds when I plant them.