I haven’t read the whole thread, but it sounds like the 4 girls have rented this house through a letting agent and each has a separate contract.
I don’t know why people are banging on about the LL, he/she has let the property through the agent to keep everything at arm’s length, your contract is with the agent. Even if you contacted the LL, they will want you to liaise with the agent, who they pay a lot of money to.
I would suggest all 3 girls tell the agent they are not paying any more rent until this dreadful girl is removed from the property, at which point they will fully pay their withheld rent. That way, they are saying “We will pay, but not till she’s gone.” The agent does have a duty of care in this case, as they foisted the 4th girl on the group, purely to fill a room and make money.
Whilst I never normally suggest parents doing things that adult children should do for themselves - in this case, you are presumably your daughter’s guarantor, so I think it’s reasonable for you and the other parents to intervene and badger the agent. It will be more difficult for them to fob you off than a bunch of young students.
I would badger the agent every day and the other parents should do the same. Stress that they have a duty of care, and that this girl is threatening their safety.
Have any of you parents actually been to the house to observe what’s going on? Or could you go and stay locally for a few days so that you can go round to the house and make your presence felt?
In the meantime, the girls must call the police EVERY time she does anything threatening or dangerous, like locking them
out.
Is it possible to find a 3 bed short let for the girls to get them through the next couple of months? If so, I would just get them moved and to hell with the contract on the shared house. Put it in writing to the agent that THEY have broken the contract by introducing a stranger to the property and by failing to give a duty of care. Threaten to sue them for breach of contract.
Lettings agents are the scum of the earth and the ones who deal with student accommodation are the worst. They know there will be an annual turnover, always more demand than supply, and they treat students with contempt.