My friend asked me to flash his phone to a K810i which is Orange Branded. Will this FS "K810_R6BC002_FS_EUROPE_3_RED52" which worked in my O2 branded phone work on his orange or do I need a seperate FS?

you only have to need same firmware for FS and MAIN
if your phone is branded or not, it doesn't matter

Thanks very much, just flashing it now ^^