Turns out the HDMI port on the monitor is a bit loose and the HDMI cable wasn't connecting until I literally jammed it in there. I'm a right and proper idiot - sorry for wasting your time Werelds. Thanks for the help! 
I can never be free, because the shackles I wear can't be touched or be seen.
i9-9900k, MSI MPG-Z390 Gaming Pro Carbon, 32GB DDR4 @ 3000, eVGA GTX 1080 DT, Samsung 970 EVO Plus nVME 1TB