I want to tell you my story, because I think it is probably fairly realistic and not uncommon, and it may help you to see that the important thing is the long game.
First thing to say is I am not in the UK, but in France, where it is generally considered harder than the UK to protect your kids.
I was divorced from my EA alcoholic ex husband when my kids were 2 & 3. I felt sick every weekend they were there. I worried how far he would go in their ill treatment to punish me.
At the time of the divorce he chose to have them only every other weekend to punish me (he got that wrong). Then when he had understood what he had got wrong, he then went for 50/50 but he lost. By this time I had got a very good lawyer.
There are 3 things to understand at this point:
Judges do not like turning over previous agreements.
They will not turn over previous agreements without there being a significant change in circumstances.
Lawyers, will generally accept flexible payments terms, and good lawyers are not necessarily the most expensive.
So, then he accused his mothers partner of abusing the children - no evidence of abuse was found after a 2 year enquiry. I supported my ex parents in law, because I knew this to be the case. But, they were my support network, and the damage to that was huge. The kids could no longer to their grandparents, even after the enquiry, as I risked being painted as a neglectful mother.
Then his mother took us all to court to get a right of access (in France the Grandparents have rights). I supported her claim, even though I knew that it was likely to have a negative impact for me. She won - so now a triple access arrangement, with my ex MIL having the first 4 days of the two week holidays in Oct/Nov, Christmas, February, Easter and one week in July and one week in August.
All through these proceedings I:
Was non confrontational, kept it to the facts and went for a narrowly defined thing.
Always always went for the interests of the children, even if it wasn't in my own interests.
Let me lawyer speak for me, practiced the one sentence she gave me to say. (I am bilingual, but stress does make you stupid).
Answered any questions I was asked briefly.
Had any and all evidence ready to hand - including recorded death threats, drunk phone calls etc. The judge can hear these if the person recorded agrees to it. He did - so the judge heard his death threat.
Get witness statements from people, in France we have set forms for this but I'm not sure how it works in the UK, but I imagine there is something similar.
What we discovered is that if we gave him enough rope he hung himself better than we ever could. We would go to court and my lawyer would read her 1 page statement, and he would read his 58 page statement and look like an utter nob. He would bring up everything that had happened all his life, every past court date, things that his mum did when he was a kid etc.
Every time he threatened the kids or me, I went to the police and made a complaint, so there was a paper trail. Every single fucking time. I never expected any action to be taken - there never was. You are building up a dossier against him. Brick by brick. It is only when this is high enough that you can win.
I found someone (turned out to be our GP) that the kids would talk to, who was prepared to do statements for the court. Finding someone the kids could talk to was a major difference for them. I have now found a proper psychiatrist who they are prepared to talk to, but it is really hard to find someone they like.
I explained to the kids that everyone would try and manipulate them, because everyone, even me - though I would try not to, had so much to loose each time.
My eldest was deeply affected so we looked at the Out of the FOG website, which has been really helpful. My kids are mature beyond their years in certain ways as a result of all this, but the only way you can protect them is by being upfront, so they know they can trust you.
I told the kids what was going on all the time. I explained that I could not go to court just because I thought it was wrong, because I would loose, but I would support them if they ever choose to, all the way through. This is something I think MNetters find hard to accept. It's not because we don't love our children that we let them go to abusive partners. Its because if we stop them going we could loose them forever.
When my children (aged 8 & 9) had finally had enough - and said they no longer wanted to go back - after he had trapped them and yelled at them and threatened violence for 2.5 hours. Then I could finally do something. So I made appointments, and it takes weeks to sort them out.
We went to the doctor, where the kids said more and she said they musn't go back. So from her office, where I now had more facts, we went to the police and they interviewed my 9 year old, and I made a complaint on her and my 8 yo DD's behalf. We came out of that with another piece of paper for my lawyer.
Then I went to the appointment with my lawyer, to tell her that I wouldn't be sending the kids back to him. She made a representation to the court for an emergency appointment, which we got a month later.
I then went to all the local gendarmeries to tell them what I was doing, in advance of my kids next date to go back to him (3 days after my lawyers appointment) for a month of summer holidays. I said I would be putting my kids out of harms way, but I would not tell them where they were. They all knew that my ex had issues. I had reported the death threat to them, but they had refused to take action (unlike the police).
Then I informed my ex they wouldn't be coming to him. And the shit hit the fan.
He made a complaint when the time came and went. I now have a criminal record as a result. I accepted that I had done something illegal, against the legal court agreement for our children, and said why. This was another piece of paper for the lawyer.
We went to court, the death threat was heard which although not relevant to the case really really helped, and the judge agreed to hear the children. My ex brought up the recorded call saying I had manufactured it, which allowed for the recording to be heard with his agreement.
The kids were awarded their own lawyer, and a week later we went back for that - just the kids one by one, with their lawyer and the judge. She took statements. She let me read them (I took scans on my phone), and then 3 weeks later we got our judgement.
The judgement was 2 hours a fortnight in a visitors centre, for 4 months. At the end of that time, we were meant to agree a new agreement between us, or one of us was to request a new court date (more costs). The children chose to have a break from seeing him. We no longer have any valid court judgement.
We are now a year on and they have not seen him again. He rings for end of term school reports and their birthdays.
The children are much happier and more stable. It took 8 years to get here. And it may be temporary, but having been heard by the judge once, they will now be able to be heard by the judge every time, which is a significant difference.
The kids also understand that if either of them choose to see him again, it does not mean the other one has to. And that I will defend their right to make a choice all the way.
Anyway, all in all, good luck.