I think it might be one of the wild brassicas but I cant be 100% and I cant find my wild flower guide.

It’s a close up of the new part of my allotment that has yet to be sorted I know what the others are but I have not seen this before

It is Chinese mustard. It self seeds very easily and as you are on an allotment it's more than likely that a previous 'occupant' grew it.
Taste a bit of the leaf and you will find it very 'mustardy', hot and tangy. You can stir fry it or simply dig it out, as there are better Chinese greens. I'd recommend Mizuna greens and Pak Choi.