All credit goes to Maka91 and vSully on this 100% Walkthrough but this Solution is intended for those who had trouble with the last collectible not registering 24/24.
With thanks to Hughes83, paraphrasing: You have to beat the game in one sitting. Collectibles will be buggy at any point, requiring you to do a second playthrough.
Don't panic! Once the game is over. Reload the Autosave checkpoint you have. It should bring you back to the end of Chapter 5. In the air vent. After spawning in. Continue straight onwards to the fan to end the Chapter (everything was already completed for me) sit through the tedious load screens, cutscenes and progress as you did previously on Chapter 6 to the final collectible. Don't pick it up yet!
Open your Memo's.
You should have 19 pages to browse through and of course multiple Memo's are on the same page. (Which doesn't narrow down too well if you're unfortunately missing multiple).
So flick through from the back to the front. Then front to the back for good measure just in case there's a "read/unread" system in place.
When the final collectible becomes ready to pick up.
I picked it up facing directly in front of the wardrobe the first time and it didn't unlock. So. I collected it from the narrow corridor side (from where you picked up the first Shoe). Facing towards the partially open door. Worked first time trying it this method.
It could be completely bogus, which I apologise if it's wasted your time but hopefully it's solved the problem and you won't have to resit through this game again!