How do we remove our little boy from domestic violence?

Hi everyone, my husband has a 5 year old little boy from a previous relationship. We've always Co parented really well with his mother up until his mother met her new boyfriend, within 3 months she found out she was pregnant which we supported her with and she always said that they got on well however Harry mentioned they argued (but everyone does in their relationship right?!).. Anyway, a year has passed and the baby has been born and recently we've noticed Harry's mum not be as cooperative.. We randomly had a letter through the door from CSA saying my husband had to pay £100 weekly for harry (we have him Friday - Monday so always had the agreement that we'd pay for whatever Harry needs in the time we have him and also buy his school uniform etc and pay for school trips etc etc).. We spoke with her about it and she said that she feels like my husband should be paying her money as well as paying for Harry when he's with us. Instead of going through CSA we came to a private agreement however we felt like it wasn't her talking, it was her boyfriend.

In the car today when I picked Harry up from school, I asked how things were at home and he said "mummy and Daniel aren't great" and I said "oh why?" and he said "well the other night he strangled her" and i was totally shocked, I told him if he was ever scared to ask his mum if he could borrow her phone to talk to daddy and we'd come and pick him up.

Going forward, what do we do?? Harry hasn't said that Daniel has abused him in any way but we don't want him even being in that situation in the first place. We don't want to mention it to his mother as she will just tell him off for telling us.. We want him to feel safe and happy telling us.

If this continues are we able to go down a legal route? Somehow get Harry away from Daniel?

There have been small things Daniel has said and done in the past which makes me questions him as a step parent to Harry.

Thanks for advice in advance!