I think these are flea beetles rather than flies, they can be a real pest on brassicas copy and paste the RHS link below for more info
https://www.rhs.org.uk/advice/profile?pid=510

Yes I also think it is flea beetle. the RHS advice is one of the best. use the link Moongrower has provided.

I'm sorry but I disagree, I think they are cabbage root flies that have hatched from the maggots that have fed on the roots. In my recollection flea beetle are very much smaller as the name suggests. I remember seeing them on cabbage seedlings and radishes when they produce serrated edges on the leaves.

I have enlarged these photos and they are definitely flies of some sort.The identifier is two wings folded over the back.

I cant get them to enlarge clear enough. I thought the shiny 'wings' might have been the shiny wing cases which is why I went flea beetle.

but either way its not good for the plant.

Agree SBG