Sorry for the rather long delay in replying. I never dared to put the chicks back under the hen so ended up raising them in the house and gave them a new batch to hatch. She's now a happy mama to 3 little babies.
I think I originally misunderstood the situation and she didn't try to kill the hatching chick. She's getting lots of extra treats as she's bren broody for such a long time to help her gain a bit if weight.