You have a couple of letters to write young lady.
What they are doing is not on and is classed by the Office of Fair Trading as:
Physical/psychological harassment
j. acting in a way likely to be publicly embarrassing to the debtor either
deliberately or through lack of care, for example, by not putting
correspondence in a sealed envelope and putting it through a letterbox, thereby running the risk that it could be read by third parties.
What you are receiving is likely to be publicly embarrassing to the debtor deliberately
Your must write in the first instance to the OFT and include copies of those envelopes, they will then direct you on what to do next.
Don't delay on this, have it ready for posting first thing Tuesday morning.