This is a helpful thread. Disappointingly we have had similar problems recently come back to the fore, culminating in being denied access per our court order. Our solicitor advised us unless there are several instances of breaking the court order they really won't be interested. Much as it disappoints I think we had reached the same conclusions about nothing will be done, so I kind of appreciate you setting our expectations.
At the moment we are just stuck with making notes of everything, and trying to help SD with dealing with mum. I really sympathise Asteria what you describe is so similar, we had hours of screaming about going to bed, 3 hours of screaming from a 9 year old - so so disturbing. It is hard being the step parent. All you want to do, every instinct in you is to protect them from this person, but this person is mum. Awful.
All I can say is what I have said to my partner, there comes a point we have to stop being protective of mum, we try not to cause further conflict but ultimately by not saying 'I don't agree that mum did that' we appear to be condoning her behaviour as ok. Instead I think we need to push more to find out what is really going on, we just get 'you know how mum is', when in reality I don't. For example, if the kids just refuse the phone then take it off speaker phone, take it into another room and say, you are pushing them too hard, you are making them upset, you need to calm down and I will try them again later for you.
The problem, if that wasn't enough problems, is that even were you to get residency, you STILL would have to deal with her for visits, so really nothing is fully 'solved'. It's a horrible situation, I hope you manage to help the kids. It makes me so sad SD is so happy here, we send her back and she comes back an exhausted sad mess again.
It's a very stressful situation, I got very upset about all of this, so I also say to you look after yourselves and be confident in yourselves. The kids need to see us be strong and calm and in control, to be their calm adult they can be confident in when sadly mum lets them down.