Ok. Here goes.
Email that I sent went down ok. He seemed to understand (although talked a lot about changing things so I felt different, not so he behaved different...).
However at about 6pm (he'd already had a fair bit to drink) he kicked off regards my ongoing insistence that I'd be sleeping separately (remember, the one I said was a non-negotiable part of me staying over...?), wouldn't accept my reasons (not that I needed one, I know...). I was all ready to go home, had DS2's jacket on, NSDH was yelling and shouting and slamming that if I left that was it for us. I thought... 
...and decided that was fine with me and carried on getting ready to leave. However, I didn't. The guilt he was piling on me about his daughters (who I love very much) worked. 
After he knew I was staying (ie I'd put DS2 to bed) he apologised and said he'd been completely in the wrong. But I was furious with him and basically didn't speak to him for the rest of the eve - i went and sat with both girls in their bedroom and watched a film with them.
When the girls went to bed I did go through and sit with him in the living room, next to him on the sofa, but he was quite drunk by now (easily 2 bottles of wine, plus cider, whisky) and it inevitably kicked off over nothing. It was like being transported back to being pregnant - stone cold sober, watching him all swallied up and giving out to me endlessly, rambling, contradicting. I tried to go to bed, he yelled and yelled at me (scaring the hell out of me), followed me to bathroom and continued. Finally went to bed (on my own), shattered (although FW watched TV passed out on sofa extremely loudly in room below where I was sleeping till 4am, so not much sleeeeeeep.).
In morning, I thought about just going when I got up. I should have. But I felt I couldn't do that to the girls. Stupid, stupid me.
He came down when I was feeding DS2 brekkie. Asked me about my plans for the day - this is a common tactic of his, forces me to come out with something which is then open to comment/ridicule/him kicking off and being able to blame me. I said I was going to go soon after brekkie - meaning only that I was going home, nothing more. He started asking about what kind of divorce I wanted, what I wanted to do about the house. Since he was talking like that, I responded, talking about a quick divorce, selling house, etc. DSD2 came in, he immediately told her that she better not get too comfy in her room because I was abandoning the family so we'd need to sell up and have all that upheaval again. Much to-ing and fro-ing, him shouting and saying horrible things to DSDs about me, saying again if I left the house that was it, me in no way inclined to stay now and crying, about to go, taking stuff out to car.
But NSDH wont give me DS2. He says I can't have him. He says he's keeping him. Refuses to hand him back.
Then he tells me to leave. Tells me he'll call the police, I have no right to be there now. Shouts repeatedly. Tries to get me to leave, I wont. Grabs me hard on the arm, pushes/manhandles me about to get me out the front door. I'm shouting and screaming, resisting, DSDs see it all. NSDH makes me go out the car and get sleeping stuff for DS2, tries to lock me out (but I grab his door keys, hahahahahahahaha so he can't). He wants me to go but I refuse. After what happened to my godmother's grandson, there's no way I'm leaving without my son.
So, i agree to sit and talk calmly gulping down my panic with NSDH after he's put DS2 down for a sleep. My plan is actually to lull him into letting his guard down then make a grab for DS2 and run for it. He says all the right things - he's so sorry, he's been awful (no shit), he's just been on the defence because he could feel me pulling away (yes, because he was being so awful!!!)
So I stay. But mainly to keep things calm so I can leave with DS2 when I want. Still shaking, I spend the morning with DSDs (who both are appalled and horrified with their dad, again) doing girly stuff. Bless them, they did my hair, make-up and nails for me! I leave as soon as I can with DS2 (after lunch, so girls are still there and I'm not left on my own with NSDH).
I know I should've called the police - I did actually text my friend and ask her to call them (because if he'd heard me calling things would have got much worse), but her phone wasn't on. But I plan to call the DV unit tomorrow.
He wants us to keep trying. I'm feeling so numb and could quite easily gloss over all this and keep going, I know I could - I'm lacking in RL support at the moment, because my family have kind of left me to it now. But I know it's time, I can't put the girls through that again, or me. I can only thank God that DS1 wasn't there. I just don't know if I've really got the strength, I feel like I'm hallucinating all this. He's being all lovely again now. I've thought maybe I need to give him ultimatums - no more drink, and find an abuser programme and go on it.
The questions I have are for bertie and hilde, about what happened when you spoke to the police about past incidents? Did they speak to your FWs? Did they log any past incidents that you mentioned?
On the upside, I am currently enjoying an M&S pizza and bottle of wine courtsey of a tenner I found in the pocket of a pair of jeans NSDH left at my flat. 
I feel that's the least I'm entitled to.