So, after looking at Darkhogg's "Reboot to {OS}" scripts for rEFInd Next Boot selection scripts I realised that that method doesn't work for me.
I could not find the PreviousBoot file at /sys/firmware/efi/efivars.
So, I started looking around the rEFInd efi directory and voila Its in the /boot/efi/EFI/refind/vars directory.
So instead of modifying the contents of the file