What is PRA GROUPS P.O box number?

[Dodo67]

I have had numerous letters this year from DVLA. I returned to sender saying never know at my address. Luckily i took pictures before returning. I am full aware its illegal to open mail addressed to someone else. September this year i had a letter with rtnso2,po box 10244, leicester, LE3 4FR. Again i wrote all over it saying never at this address. Now for 4 weeks a supicious man dressed almost like police has been knocking my door, and only my door and parking out of view around the corner. So because im a vulnerable adult and dont answer door and this is happening every tuesday. I googled po box. And PRA group/Aktiv Kapital came up. Being stupidly law abiding and never opening letters i had no reference number to quote when calling them. So this has led to them telling me they never sent the letter or received it back. I told them i photographed it. Now they tell me its not their po box number. I am so pleased i took pictures of the letter. There is more to this story, but where do i stand legally about this? I also asked was it a debt collector knocking my door each week and the reply i got was we dont send people out. Lies i said your a debt collection company if course you do. Now i hope its not a bailiff because the person on the letter is not even the same sex as myself and im sick of having to prove myself to you let aline to a bailiff.Why is it i have had to contact them repeatedly and for something thats nothing to do with me. Can legal action be taken against these incompitant idiots?