A very complicated situation, you could probably do worse than get the advice of a lawyer. Of course your Dad should pay his way, and it should be for all the costs of his bungalow, he is taking the mickey! it would have been sensible to get everything in writing first, but at least you can find out where you stand legally.
You can't do anything about his will, that is up to him, he has put £40,000 into the renovations and nothing else, and not leaving you anything further would indeed be unfair, but I don't think you can do much about that, other than call a family conference and put everything on the table to everyone.
If he is gambling away the proceeds from the sale of his house that is really sad, but unless you can get him to seek help, again, not much you can do. I am presuming there is no mental state deterioration going on, in which case you should be seeking a power of attorney quickly, if you could persuade him.
I feel for you, you have bent over backwards to make his life easier, and he has taken advantage.