Answers
File One:
Click the book cover image, it will take you to a page where the book cover is built in ASCII. Zooming out on your browser will show you the missing letter: 0.
File Two:
Right click the application page and select 'view source'.Viewing the page source reveals the second letter, which was commented out of the code: S
File Three:
Hovering over the 'Utilities' icon on the Necronomipod reveals a 'Panic' button. If you click this, it shows a clue. The answer to who said the quote was Arthur C Clarke, so the third letter is 'A'.
File Four:
Signing the waiver will give you a clue. Go back to page one and click on the word 'extradimensional'. This is a hyperlink that will take you to a hidden page that gives you the letter 'S'.
Find the Vault:
The clue for this one was in the wording 'so close to the end'. Applicants had to change URL to read 'endoftheline' rather than 'enboftheline'. This takes them to the vault where they can enter the password 'osas' (official secrets act signatore) to access the application form.
Thank you all for your applications and for taking part. The Apocalypse Codex is out now - what are you waiting for?