You can get a copy of the will from the probate registry for a fee of 6 pounds. It is public domain. You will need the date of death and obviously full name.
Once you have the will you will know what is in it with regard to living in the property and normally it would say that he could live in it provided he maintains it and ensures it and does not remarry or cohabit but you need to check the will.
If there is nothing in the will about that, and he is free to have whoever he likes, and go in the house and stay over. Just because someone stays over does not necessarily mean that they are cohabiting if for example it is live in care.
Unfortunately, what your father is experiencing is not uncommon in circumstances like this. It is potentially abuse of a vulnerable adult if the ultimate beneficiaries of bullying him to try to get him out of the property. That is an issue for social services.
If you get a solicitor to write to them threatening reporting the matter to social services if they do not leave him in peace they may leave him alone. They cannot stop him having people to visit or stay over unless there is a stipulation in the will.
Nor can they come and go as they wish if he doesn’t want them to because that is a breach of his basic Human Right to a private life.
If they persist, then it would be necessary for your father to make an application to court for an injunction to stop them coming near him or the house.
There are lots of things that you can do.
Can I clarify anything for you?