Why did you let a stranger into your house?
Why did you even speak with Capita?
Those letters are just computer generated, there is no "investigation".
Do you usually fall for scams?
You let a stranger walk into your home to "check" something!
He could have been anybody.
If a random man knocked on your door and asked to check the contents of your fridge would you have let him in also?
I hope you didn't sign anything.
It's a common tactic of the Capita sales people, they look around and than ask you to sign their paperwork to "confirm that you don't have a TV". This paperwork is a prosecution witness statement called a TVL 127 form, they basically write out a statement admitting that a TV is being watched illegally and then get gullible people to sign it. You end up in court, they get their commission.
These scumbags aren't decent people, they aren't nice or polite. They prey on the vulnerable and poor.
You were really silly to let that man into your home.