Worth noting my home button doesn't work (although the fingerprint scanner does).
I'm able to access the 'Warning: A custom os can cause critical problems..' screen by holding down my power button and either of the volume buttons, but once on the screen I can't continue or cancel using the volume buttons, the phone simply doesn't respond. To turn the phone off again I have to hold both volume buttons and the power button for an extended time.
If anyone has any ideas, that'd be much appreciated.