Martin, I used to be a conveyancer and I know of no law that says that a relative moving in with a house owner becomes a part-owner of the house! Social Workers are probably not experts in property law.
If the father had contributed to the cost of the house, his share should have been registered with the Land Registry. If he had made a gift of the money to help his son buy the house, I think that would have counted as a life-time gift and my first reply would be relevant.
I gave both my daughters a considerable sum (£20,000 each) as deposit on their houses, but I did not want my name on the deeds. I was only 42 and in good health, and in any case I had no capital left. My employer was providing me with accommodation abroad, so I used my half share of the matrimonial home to get my girls on the property ladder. (Unfortunately, when I returned to England I had to start again from scratch with a mortgage!)