Nightmare housemate

I have recently moved back to London and live in a houseshare with 3 other people: 2 of which are very decent and the third who is a young bloke (22, whereas I am 28) who has pretty much moved his very loud and disruptive 19-year old girlfriend into his room, although when questioned he insisted that she had a place of her own. The two of them are in the house most nights these days and the girlfriend even does her washing (at 90 degrees, might I add, though she knows we are an eco-conscious house, and she contributes little to the electricity bill) in the house and hangs her smalls in our lounge. He has now taken to being up with his gifrlfriend, running in and out of his room, making a noise, playing loud tv and generally creating havoc in an otherwise quiet house until 1.30am on weeknights. What can I do? My other 2 housemates refuse to stand up to them as they are worried he will move out and leave us in the lurch. We live in a house where we have all signed one lease, and so are all responsible for the rent but the landlord has our details and thus it would be quite difficult for him to skip the place without paying or finding someone to replace you. Please help!!!!!