You can't leave him because you are addicted to him. Have a look at CoDA - codependents anonymous. It certainly helped me to address my addiction to my very abusive husband. It is very common that people are addicted to the abuse/the addicted partner. Sounds mad but very common and I put my hand up to that.
As you are in the UK, get along to the nearest Freedom Programme - click the 'Search' button to find your nearest group. You will meet women in the same position as you who find it hard to leave an abusive man. It may sound cheesey but the fact that your dad wasn't around when you were growing up probably has a lot to do with why you are addicted to your H. The Freedom Programme addresses the various ways these men encourage our addiction and in a structured programme looks at some of their tactics, supporting you to get free.
womens Aid is a brilliant port of call - 0808 2000 247 (24/7 supportline - call between 7am - 7pm as the lines are very busy during the day) who will support you emotionally, practically, legally to take steps to get this monster our of your life. yy he's lovely when he's lovely but you can't risk having him in you and your childrens' lives any more if he does things like punch you in the face when you are holding a baby.
when you took him back you were sending the message that what he had done before was acceptable - that's how he sees it. He won't change. Abusive men are good at making promises because they know exactly what you want but won't give it.
He is now psychotic and you must inform the relevant authorities that he is on the loose in your life eg Womens Aid, the police. You must get you and your children protected and do whatever it takes. If you continue to take him back, social services will take your children - it's as simple as that. they've seen it all before - women addicted to abusive men - but their job is to protect the children. Womens Aid and the Freedom Programme will support you to take steps to get him out of your life and to address your addiction to him. A counsellor at some stage will help you to look at how you have become addicted to someone who most people would run a mile from. As I said, I was like that and countless people are, so don't feel ashamed, but do be proactive to protect your children first and foremost but also, of course, to protect you and get you on a more healthy footing.
Good luck, plenty of us have done it (got away) and lived to tell the tale.