The injured hen is actually healed now, but I am sure that he will hurt her again, so I am keeping her separate. The other hens spend the mornings with her so that they can get their egg laying done. They get very distraught when separated from the cockerel, which is why I caved in and put them together again.
I was naively hoping for an 'easy' solution, but I think it is clear that he should not be let near her again. And that she should not be punished for his bad behaviour, which it feels like what I am doing now, albeit unintentionally.
It's a bit logistically challenging keeping him locked up and them outside because of my setup here (and their need for access to the egg laying box), but it can be overcome, so I will make a plan for now.
Thank you everyone for your suggestions, it has been insightful, and I have a bit of a better idea about what should be done. I am still new to all of this, and astounded by how ruthless animals can be, a real eye opener. I have first hand experience of a guinea fowl male (my tame baby) that turned into godzilla, and still haven't recovered from that. I'm not up for a repeat performance.