this is caused by the leaf miner. you may not have the sae problem this year. try growing it in different places in the garden

thanks, maybe I'll buy some new compost as well.

I think you probably also had flea beetles. They are small and black and jump like fleas so sometimes look like flies. The make lots of small holes. You can still eat the leaves but they don't look very nice. If you cover with fleece when you plant they won't be able to get to the leaves. However I am in danger of covering my whole garden in fleece...

Sorry for late reply but thanks Marshmallow. At least I can still eat it then. I did wonder and actually threw quite a lot away last year. Will try the fleece idea.