I unrooted today, thinking it would let me update to Lollipop, still says custom on my status page so I guess I have triggered the efuse and can't use the system update facility.
Downloaded the GT-I9505 firmware from sammobile, have backed up all contacts, media etc.. to SD card, must have gone to a dodgy site for Odin as installing it just ran up a load of spyware and wiped all settings and plugins from chrome on PC, oops!
Guessing I just need to get a proper Odin, boot with power, home and volume down, apply the image via the PDA box and apply it, then boot with volume up,home and power held in and then set it to format?
Never done this before so excuse any dumbness, just don't want to brick the phone or cause myself any more issues, phone has been on 4.4.2 for 2 years now and NEEDS a wipe, so slow right now!!