Very little point in writing and you certainly would never state anything about owing a debt.
All that happens is that some DCA's pass on a list of debts to local collection agents working on commission. They will knock on a door asking if Mr x lives at the address. You would never confirm your name at your doorstep anyway. So you just shake your head and close the door. Chances are you would not be in anyway. They don't spend too long trying at an address, as they need to visit as many as possible, given they are on commission.
ignore and stop worrying.
Don't touch them owe me £500 since January 2019 make excuse after excuse. Seem they always have software problems sending money out. Keep saying they will call back or email nothing been chasing it now for 6 mths the phone staff always have the same banter we will chase it up and get back to you then nothing!