Spouse Visa UK staying with parents help

Good afternoon all,

I need some advice I am currently in the process of doing a spouse Visa for my wife to live in London with me.
We are looking for a place of our own which should be sorted by the time she arrives but for the application I will be putting my parents address as we can stay here in case of any delays.

My parents have been renting from the local council for the past 37 years can anyone please tell me what information I will need.
So far I have a council tax letter and a letter stating they are happy for us to stay with them.

Now because they have lived there for 37 years we are currently having difficulty finding a tenancy agreement. Can anyone advice on what else we can provide and what info is necessary?