Thank you so much for your replies and support. I was a bit dubious in joining a forum and telling people about this, but I really wanted to hear from others in the same situation, people that understand it more.
I realise I should have reported him for the sake of my mum and myself. I know it would really hurt her if I did, but at the same time it would protect her and be better in the long run. I stayed with my fiancé's family for the night after he had hit me (they're some of the only people that really know about the situation). When I told them I was thinking of reporting him they told me it would be best not to because it could ruin my relationship with my mum. Now I really wish I had reported him and have vowed to myself any more violence and I will.
I've called the police a few times in the past, and every time I have they don't seem to take it seriously, they just give him a slap on the wrist and off they go. In fact they seemed to get frustrated that they kept getting calls from us about him.
It's just so horrible seeing him constantly put my mum down. When she gets upset and cries he shouts at her, telling her she's got nothing to cry about because he's the only one going through hell. He tries to turn people against each other as well. For example a couple days ago he said to me that my fiancé doesn't love me, that my friends all hate me etc. He just seems to take joy in putting others down.
Thanks again.