...."There's no reason whatsoever why the post wasn't put through the letterbox".......
I can think of a reason. What about plain old human error? I imagine the postman simply dropped your pile of letters and did not realise the mistake. Or maybe some oik stole them from his sack without him realising.
Either way, there appears to be nothing malicious on behalf of the postman. If he had a vendetta against you, he would simply throw your letters away.
The fact your details are on the letters and can easily be traced to who "should" have delivered them, and this is a one off occurrence, I fail to see why you need to pursue the complaint.
Don't get me wrong, Lindylou, I am amongst the first to grumble and complain, but this case simply seems like an unintentional accident with nobody to blame.
Can you not let it lie?