Can I stop my Ex's violent new partner from seeing our son?

[schnell1177]

My partner and I have recently split up but are still living in the same house. We have a 2 1/2 year old son. I discovered he has been cheating on me with the woman he was in a relationship with before we got together. They had already separated before I met him. He was with her for a number of years and during that time she used to hit him on quite a regular basis. When speaking to me about it on one occasion he compared himself to a battered housewife who tried to leave a number of times but kept going back to her like she had some sort of hold over him. His mother knew about this and she absolutely hates her for it. Apparently she has never hit anyone else but I've told my ex she will not be allowed anywhere near my son. I can't risk her being violent towards him and I can't be 100% sure my ex would be able to protect him from her when she has previously had such a hold over him. Would I be able to apply for a Prohibited Steps Order to stop her being allowed near my son? I don't have proof that she hit my ex, just what he and his mother have told me, would this be enough?