Hi,
I have a bit more info. I tried the solutions to the "greyed out none option" problem on Google, even though mine is missing rather than greyed out and it didn't fix the problem. In theory, if it had been one of these problems, the "swipe" option would also be similarly affected so I'm pretty sure that this is not the case.
Specifically I tried clearing my credentials (i had no user credentials anyway). I removed my exchange account. I cleared all the checkboxes for everything in the device administrators.
I havent tried a full hard reset yet (because I'd have to save all the data on my phone first).
The option is still entirely missing.
Maz