Hi GUP,
Re your comments:-
"My kids are 7 and 4 and dp is not their dad, they adore their dad and have a good relationship with him, although they are close to dp too".
Good to read that they have a good relationship with their birth dad - long may that continue. They need a caring male role model/mentor in their lives, your partner does not fit the bill in that regard.
"This is not an excuse, I know you are right about them seeing more than we think, but in a way that makes it easier to think about splitting. They will be hurt, but not as devastated as if he was their dad".
They may secretly be pleased that he's gone because their Mummy is happier as a result of him not being around daily.
"He's kept a low profile today and to be fair hasn't drunk much of the box, Maybe he was trying his luck or making a point - who knows".
I would say both, its selfish isn't it?. It's not relevant tbh that he hasn't drunk much from the wine box - the fact that he's bought it when he knows he has a drink problem is bad enough. You sent him out for orange juice, he came back with this. He clearly cannot be trusted with any responsibility.
"I obviously haven't hit my rock bottom with him yet"
No you have not. But you will.
"but I can see it looming in the future somewhere. Every time I try and end it something stops me and I can't quite do it".
Why is this, have you given this real thought?. You need to think about this and why you are holding back. Perhaps you feel sorry for him and like many who grew up seeing alcoholic parents perhaps have a sense of responsibly for him. Many children of alcoholic parents become super responsible as adults for others.
"I suppose because I love him and I'm not ready to give up on this, I wish to God that I could switch it off and not feel anything for him, I really do".
Do you honestly think that if you walk away you will have failed him somehow?. He has failed his own self, he has no-one to blame but him. You are not responsible for him and his actions; you can only change how you react to him. After all, he made a conscious choice to buy the wine box.
I think you need to start getting angry with him - he deserves no less tbh with you. Be brutally honest with yourself here - do you really love him or do you at heart feel sorry for him?. What's there to love exactly?.
What are you teaching your children by loving someone like this, this man who needs no real encouragement to mess with your head and beat you down in an emotional sense by casting aspertions on your parenting skills (not least of all to comment about your Dad). That's not love is it?. It's emotional warfare. You're doing all the giving and he's doing all the taking and gives nothing back to you but pain.
He neither knows or cares what he's doing to you.