I wasn't able to get @SIMON's method to work unfortunately, but I followed @smx's tutorial to roll back the firmware for my LG 55UF850V from 4.05.60 to 04.00.90 and that worked, up until the point where I tried to run the update on the TV. The TV was able to find the available update (coming up as 04.05.61 on the tv), however when I clicked "update now" it said update failed immediately, giving the error message "Update failed because SW version is lower than TV SW version". Has anyone else had this issue and been able to fix it? I think perhaps it might have been because I was trying to update using the usb version of the .epk file (that's the only file I could seem to download on the lg support website) rather than the nsu version, but I can't seem to figure out where to download the nsu versions.
Is anyone able to advise on how I could fix this issue? Any help is greatly appreciated because my TV is running very slowly on its current firmware version!