If someone has mentioned this I didn't see it.
Given the scratches and bite marks OW knows he is married and wanted to be sure you knew about her. Especially as he says her met he while you were separated. Perhaps he told her about the inheritance and that he needed to stay for a while so as to get half in the divorce. She may be trying to precipitate another separation.
But as there's other family money he may be playing both ends. He strings her along and keeps her on the side while staying with you. He took your son last time and may try to use that as leverage to keep you from throwing him out. After all, if he can keep you sweet, you might buy him a Porsche and he can live comfortably while waiting for the next inheritance to increase his half.
If it were me, I'd make sure of the legalities before ANYTHING ELSE.
Don't text her.
Behave as though you've had time to think overnight and are going to let it go.
Don't act lovey-dovey because that's not believable. Just stay calm and be as normal as possible.
Get legal advice as soon as possible. I hesitate to say shark, but try to choose one who has the reputation of getting good results for his clients.
make the appointment first thing tomorrow, if possible.
Make them with several if you don't know who to see. You'll get a free consultation with each.
I don't know if it's true in the UK, but in the US once you've seen an attorney, the other party cannot use that person. Limit his choices.
And all the while, be calm. Be strong. It will be hard, but as someone else said, it will help your case to keep your son.
Try your hardest not to engage with him. He'll probably try to smooth things over and I'd let him think he has. After all, he's been lying and got away with it for a little while. Now it's your turn to gaslight.
I know it will be hard, but you can do this. If you need to scream, get in the car, drive with the music up loud, and scream and cry until you get it out. It works.
Do you have a ride-or-die person IRL? Someone who will have your back and keep your secrets?
Keep up this thread. You'll get lots of support here.
Be strong. You can do this.
