No I'm not surprised this bloke as also threatened your daughter and she is scared of him - I think I said I thought this would happen in my first post. I am a retired social worker (30 years in all- in LA Children's Services) and have seen this situation more times than I can remember. These women in violent relationships somehow become paralysed and are unable to stand up to the violent partner and are dominated by them, and in the end their fear prevents them from leaving, and the more afraid they are, the more dominant the man becomes, so it's a viscous circle really.
I am very concerned about your GS - I think it is only a matter of time before this man harms the child. I know that might sound like I have a "crystal ball" but I know how these men operate. Violent men are really scared inside and very insecure (and they have almost always been the victim of violence in their childhood, so they only know one way of parenting) and so they become very possessive of girlfriends/partners - they need to own them and they will usually do something to get rid of the girlfriend's support network (parents, extended family, friends etc) as they don't want anyone interfering with their dominance of the partner or trying to turn her against him. This does sadly very often extend to the child - the violent man sees him as another man's child and his jealousy about this causes a rage in him and he takes it out on the child. He also gets jealous of the mother of the child being loving to him, and this also causes jealousy and is another reason for violence being perpetrated on the child. Even if this doesn't happen your GS is highly likely to be suffering from emotional abuse as he will be aware of the violence of this man towards his mom and the tense atmosphere, if nothing else.
Is your daughter living with this man? You say you "kicked her out" - so where is she living and in what conditions? Are you still able to see your GS - and how old is he? I think it likely that your daughter will not be letting you see your GS (and I see she is already threatening that) and if this happens it is a big warning sign. That is what usually happens in these situations.
It's difficult at the moment but an offence has been committed by this man using "Threatening words and Behaviour towards you" but if you do report it to the police, you have to agree to press charges. Although I think you can just get it recorded for possible future use and I urge you to do this. As I said before the fact that police have been slow to respond to calls from women afraid of violent men, has been thrown into sharp focus lately and so they may take you more seriously.
As far as Social Services are concerned, they will not be able to act unless someone makes a referral to them of witnessing the child being abused /neglected and they consider it serious enough to go out and visit the family. Dependent on the child's age they may just contact the health visitor or if he is of school age contact the school to see if they have concerns. You say you live in London and so workloads for social workers are going to be very high and they have to prioritise. Even if they do go out, it is often the case that the bloke will tell a convincing story and the mother is too afraid of him to tell the truth. The real worry is that when women get really afraid of partners they fail to protect the child and that is when the child gets hurt. I'm sorry - I don't want to cause you unnecessary worry, but I think you need to ensure that you see as much of your GS as possible and monitor the situation. Is there anyone else in the family who could do likewise? What about the child's father - does he have contact with him, or was he one of the other violent partners.
I wish I knew why women move from violent partner to violent partner but I don't and I honestly don't think you should blame yourself. I've worked with many families in this situation and the mothers of the daughters in a violent r/ship are good, hardworking mothers who have always done the best they could for their children, just like you. If you had been an uncaring mother you wouldn't be worrying about your daughter and GS now would you. I've met plenty of people who don't care about their adult children or their grandchildren.
Do you have anyone in whom you can confide?