Here is a genuine story - the man who rented a cottage from a friend of my daughter died very unexpectedly. He was not married and had split from his girl friend a year or so ago. He has no children, but siblings here in NZ. He had been telling his landlady for some time that his girl friend had been a gold digger and she wasn't going to get anything further out of him. He was only 48 when he died (heart attack) and had not made a will.
The former girl friend has now turned up saying she is his sole heir and he promised to leave her everything. She said that as they had lived together for a couple of years, she was his 'de facto' partner.
I think NZ law is the same as British, in that unmarried partners, and especially ex partners, would have to prove that they had contributed in some way to the home/capital/ etc. I will be interested to see if the siblings are able to stop the girl friend getting hold of everything.