Beside myself with anger

That being said, Rossie, I've been rereading your post and a few comments :jumped out at me:

"...they defaulted on the last payment of rent, therefore partner had to cough up once again."

"Am so very angry, not just because of the money but because he is so stupid to stand as quarantor... and I feel that they have got one up on me, spoiling my happiness and saying 'we will always come first'.

"At the moment I want to demand that he leaves her out of his will... I don't want the money, he can leave her share to his son, am just so angry that she and ex wife have come between us yet again."

No, they have NOT come between you, at least, not intentionally. They put their hands out for money which they needed b/c of their bad choices. No offense, but they probably weren't even thinking about you at the time. And they don't know, surely, that you think DP was "stupid" to become guarantor or that you feel they've "got one up on" you, etc. You are feeling this way, unfortunately, b/c of your own reactions to the situation, and maybe, somewhat of an inability to deal w/ the fact that he has been married before (maybe not, of course). However, the wish to make your demand seems to have come only "at the moment," so hopefully, it has passed. Please don't do anything in the heat of the issue. IMO, that's never a good plan.

Oh, also, Movingon, as for my own father, he and my mother had reciprocal wills, etc. But if they were divorced, as in the DP's case here, and if he were to cut me out of his will, I would be very hurt. And if he did it at someone else' request, I'm sure I'd feel even worse - whether she were his wife or not. The court would see a difference there, surely, but, as his DD, I wouldn't. All I would know is that another person influenced my father against me.

Also, I understand that cohabitation is not on a par w/ marriage in a court of law. But we're not in a court of law here, and IMO, it's harsh to brush off 18 years of a relationship, including cohabitation, as 'just dating."

As the OP is about
A: how her partner spends his money and
B: how his assets are split on his death

It may be harsh but it is true. She has no more of a say in his finances than anyone else, with the exception of any joint ventures they have paperwork for. And if she has relied on him and his family considering her differently despite him not giving her any legal protections or rights, then a harsh wake up is needed before things get worse for OP.

OP please dont pay towards any property that isnt in your name, including (him: I pay the mortgage you pay the bills/upkeep type "splits")

Please keep savings in your name. Dont take on joint debts while you have no rights to his assets. And prioritise your own pension if you are not yet retired.

Or marry. And if he doesnt want to, then do the previous 2 parragraphs times 10

You have no rights to any of the life you have built together unless you have made legal agreements pertaining to the aspects you intend to share. And I do not mean him saying he will will it to you, I mean him putting your name on things you contribute to or things he says are joint.

That would be fine if you were on the same page, and both wanted financial independance and no expectations or duties to each other: just each other's company. As such, the guarentor issue shouldnt affect you.

IMO, notanan makes some good legal points, OP. In fact, instead of worrying about what money his DD is or isn't getting, I think you need to think more about your own situation and how to protect yourself financially. You and your partner also need to make decisions, if you haven't already, about what happens if one of you goes into the hospital, especially, say, if it's a matter of critical care. If one of you is in critical care, in other words, does the other get to visit them or only "next of kin" (in his case, his DD and DS)? Who gets to make medical decisions if, unfortunately, the ill one cannot? I don't mean to worry you, and these may not be immediate concerns. But my point is you and he may have other things to work out besides what he should/shouldn't do about his DD.

Why haven't you married, OP? You would be in a much more secure position if you were married. Do you have your own money and financial independence?

Don't be jealous of your partner's daughter, try to build a good relationship with her. You risk losing everything if you try to come between father and daughter.