As far as that switch, it's typical of an abuser.. you find yourself walking around on egg shells not knowing what your next "mistake" will be, only knowing that sooner or later you'll do something and it will set him off.
If your mom wanted to leave she could - and she could today. My thoughts? She doesn't actually want to...
There are shelters for battered women in almost every western country, and from your usage of the word mum it's my bet your somewhere in Great Britain.. I'm sure Great Britain has a shelter for victims of domestic violence..
A shelter will provide a place for your mom and any minor children, and help her with an attorney or other means of legally getting herself out of the situation, as well as they will help her with getting a job and an income to get herself on her feet and help her with getting any government help as well, such as food stamps and aid with housing.
You need to find one - they don't give out the address they are located to protect the women they are helping, but they will give out their phone number, any police department will have it..
Get it, have your mom call them.
If she won't call - she's not ready to leave and it's not your job to get her ready.. your not her parent. It's your job to do what you need to do for yourself and to keep you safe.
That's my best advice.