"In fairness to the post office"? In the first place, your previous packets were delivered correctly to my address but without any requirement for a signature (it's a retirement complex which may be the reason?).
The packet that went astray was delivered to the WRONG address and the "signature" was nothing more than a squiggle, and they did not call at my flat subsequently to investigate. Someone, somewhere, "signed" for your parcel at the wrong address, and I don't believe it was a flat here as people are very good about popping incorrectly delivered items to the correct flat. There is no-one in my flat to sign for stuff apart from me.
I'm not in any way getting at you as you did everything right, but the PO clearly lied to you. What's more annoying is that though it only cost £15 it was an item I wanted.