Common law husband rights sale

Hi all,

I know that there is no such thing as a common law husband/wife but it
was the easiest way to express the nature of my post!

Here goes:

My father lived with his girlfriend/partner for 17years but never put
his name on the mortgage (he moved into her house after her husband
moved out). However, he has paid easily half (probably more actually)
of all the outgoings over those years as well as driving her to and
from work every day (despite having to get to & from his own place of
work) & everywhere and anywhere she wanted to go at weekends. He also
spent £10,000 of his own money on a new kitchen she wanted so has
definitely paid more than his fair share as an equal partner. In fact
she wouldn't have been able to live there once her husband had left her
if it wasn't for my father paying the mortgage for her in the first
year they were together.

Well, she died last year and left a ludicrous & completely unfair will
leaving my father absolutely nothing & stating that he is allowed to
simply live at the house until his death at which time the property
should be split between her sister, son & daughter. Even items that
he'd obviously gone out and bought himself (like gardening tools) were
mentioned in the will as being allocated to certain people! The problem
is that he is very ill himself and doesn't want or need any sort of
stress so hasn't pursued a case to contest it. I think he also doesn't
want to upset his late partner's daughter as he dotes on her children &
is pretty much their second grandad. The only thing he has done is to
tell me that there is a caution placed on the property stating that he
is owed a share and that this would pass to me in the event of his
death. His late-partner's relatives (who are already very comfortable
financially) know of this and have been very nasty about the whole
thing and told him he'll get nothing & that the caution will die with
him. They've already received the life insurance pay-out plus all of
her savings and my father has been left with absolutiely nothing - just
his disability pension to survive on. It really is a complete disgrace.

If he was just some random lodger who had just lived there for a few
years I'd understand but they know full well he was her partner and
welcomed him to their family. He has paid an equal or greater amount
into that property for 17 years & I'm gobsmacked and incredibly angry
that as soon as his partner died & there's the scent of money in the
air they've become so vile & have apparently lost any sense of
morality. They really have been very nasty.

So my question is: what do I do when my father does eventually pass
away? Do I have a case to pursue here? If so, what sort of percentage
are we talking about? I just want some justice, and even if I ended up
with virtually nothing after spending most of it on solicitors fees at
least I'd have shown these people that you can't treat people this way
and get away with it.

Looking forward to your thoughts on this matter.