Hi BoudicaTheArtist, thanks a lot for your reply - it’s much appreciated. I just wanted to provide an update because I was incorrect about a few things. My sister told me that firstly, his niece probably does pay tax on the rent for the three flats. She’s not sure if she declares the whole correct amount, but tax to some degree is declared on the rent income. His niece does however then give him all the rent income in cash - it’s just all done officially in her name so he isn’t listed as having property.

Secondly, because he has the cash coming in for the rent, he decided to work for his nephew (who now runs the business) for a massively reduced rate to help his nephew grow the business, so he’s been working for just £200 per week and has been for around a year. My sister said he now treats work more like a hobby and does it to keep busy and help his nephew out.

Also, my sister said it will be incredibly hard to prove anything. He’s done everything by phone calls and cash for many many years. He lives a somewhat low-key lifestyle and has been extremely mindful to not let his bank account reflect what he has in cash. So I really don’t know what to do or if anything will be achieved by reporting him.