I remember being taught all of that so that's exactly as it is Frank. The printed postage label is top right, where a stamp would otherwise be, the Signed For sticker is below that and the address is clearly written in the middle. On the back, and what makes it even more ridiculous is that I actually wrote "sender:" followed by my name and address.
I called Royal Mail and the woman I spoke to was passing the blame onto the Post Office. As I told her, the PO just sold me the service, RM (and their system) are the ones who've failed to read the envelope correctly and delivered a Signed For letter not just to the wrong address but failed to get it signed for (and if they've gone by my tiny written address on the rear then that side has no postage on it).