Thank you all for your comments, your replies are really helpful
I have considered al anon before, as there is a meeting that's held 5 minutes from were I live. I've just been too scared of going, I had really bad social anxiety disorder in my early teenage years and although I got treatment and am a lot better these days, I still get a bit panicky in face to face social situations with strangers. I might give it a try, but I really don't think I could convince my mum to go too. Definitely worth a try though.
My mum never used to give into him too easily, but now she doesn't even put up a protest, I know it's not her fault but it's really frustrating sometimes. Around a year ago she told him if he went out and bought more alcohol, she wouldn't let him back in the house. So when he came back with a large bottle of vodka she locked the door. After about 10 minutes of him pleading with her he started trying to kick the door down saying he was going to kill us when he gets in. When that didn't work he picked up a garden chair and tried to break the windows with it, at this point I locked me and my mum in an upstairs room and called the police. My mum was in hysterics and couldn't stop shaking. I think after this she became scared of him, as she just stopped putting up any kind of protest against him.