Galaxy S6 Edge unresponsive Download Mode Screen 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.
I found a way around this by using a utility called 'Samsung 300K Tool', it allowed me to put the phone in download mode without rebooting.

EDIT: After recommendation by beeshyams.
Googling 'Samsung 300K Tool' after seeing it in a video I was able to find it very quick, however it seems Mediafire has deemed the program dangerous, I scanned with Malwarebytes but proceed at your own risk. I also can't guarantee this will work for your phone but it worked for my S6 Edge. I've uploaded the executable to TinyUpload, it can be found here:
http://s000.tinyupload.com/?file_id=06981153934260620870

First make sure your phone is on as normal, plug in the phone and open the utility, after given the go ahead by the dialogue on the right, click the image of a connector on the left, wait a moment and your phone should be in download mode, run Odin as normal.
Hope this helps someone.