Step-father is interfering and causing problems between my Mother and myself. Any advice?

Does he also suffer from dementia or other mental illness?
It sounds as though your mum might be better served in a facility. That would serve the dual purpose of keeping both of you safe from him, as there would always be others around if he visited. Something to consider.

Did you move in with your mum?

I'm not sure I understand. If your stepfather moved out and no longer participates in the care of your mom how does he still have regular access to the two of you? Regardless, if he comes around and becomes physically threatening call the police then file for a Personal Protection Order to keep him away. The police can tell you how to go about getting it.

I have the same question as Eyerishlass. And my first thought also was to consider getting a PPO against him. If he's already threatening violence, he may take it. Document each of these threats to cite as justification in the Petition for PPO.

Or contact the local police to ask for advice; that will lay the groundwork for documenting your concern about his interference and potential violence.

I'm wondering as well what brought him back. If your mother has some assets he wants, that may be his motivation.

If he threatens you, you call the police. They will remove you if it is his house.
They will remove him if it is your house. So which is it?

Sounds like he is a lazy sociopath. Read "The Mask of Sanity". If it opens door for you get professional help dealing with the situation.

I forgot to state as I think I want to ignore this fact that mum still chooses to see this man. Before mums diagnose they kept getting together and breaking up. As neither of them would change to make for a better relationship. And it is hard to want to change if it's not you who has the problem but the other person. After a lot of therapy on my side I see that they enable each other. That is the crux of my problem as well as the fact that mum forgets the fact that they have problems and when they are apart their relationship is perfect, but when they are together not so happy families. And when they are apart it's my fault for keeping them apart which my step fathers manipulations encourages. When all I want is to keep mum happy and healthy. We all get on well with me ignoring him mum and step father seeing and ringing regularly until I do something to offend him. Which could be taking mum out for a drive without him knowing and he is trying to ring her. I want to keep looking after mum as she really doesn't need to be placed into a nursing home yet because she is easy to look after. It's him who can't let go of the control to ensure mum gets the best care.

Also it's my mums house and I moved in my Aunt has power of attorney as I knew what issues I would have if I was and I didn't what that as well as the pressure of mums care. As I've been diagnose with depression and anxiety in the last three years. Which actually comes from my relationship with mum. But I've conquered that battle and still have therapy to keep it under control. I had to help mum not for but for my own sense of peace/ self worth/or whatever the h*ll it is, but it was for my own peace of mind. And I have seen how well I've grown and accomplished in the past three years nearly four. I'm getting in touch with a lawyer to find out my rights. Its just good knowing I'm not the only one struggling with like minded issues. So thank you for all your responses, it helps.