So Challenge 5 is a massive anticlimax - there it is, right there. I don't know if the page sets some kind of cookie, or only reveals itself if you do the preceeding challenges or something, but anyway. Done. http://canyoufindit.co.uk/secured and the answer is secured.
So that's how I did it, I'm sure there's a thousand other ways, but I definitely learnt something, and if you're still here maybe you did too!Hacking CanYouFindIt(top)
Before I put the time in to try and solve all of the challenges, I had a poke around in the source of the website, seeing if there was some kind of back-door.
With a little bit of hackery, I was able to reveal the hidden "enter your contact details form" and hit submit... At which point the website said "DENIED" and told me to go away and do it properly: