Hi, and welcome. I think we all can relate to your experiences and I know it can be tough at times.
Any tips on coping or encouraging private care ? (He won’t entertain this at the moment) I know you can’t force someone to be treated
I've heard about DBT but have no experience on it or care in the UK. You're right that you can't force someone to do something they don't want to. But you can change your own thoughts and behaviors in a way that make it easier for you and will likely be less enabling to his BP behaviors. If you haven't already, I recommend reading Stop Walking on Eggshells followed by Stop Caretaking the Borderline or Narcissist to get into that mindset.
H is now desperate to set up his own business however.
Are you financially together or separate? It would be best to limit how much money he puts into the business to protect your financial well being.
it was myself that gets the brunt of a huge rage/tantrum ‘ I’ll do the cooking in future’ he blasts out
Maybe tell him calmly if he doesn't like your cooking that he can make something himself or pick up dinner somewhere. You don't have to suffer behavior that you find unacceptable. The same patterns will repeat until you break them.
get in my car and park up for 2 hours at a garage late night. I have no food and drinks or anything
Keep a bug out bag with food, water, medicine, and anything else helpful.