Unable to raise t/e flaps between 5 & 1 after t/off except on alternate system. Company decision to return a/c to lgw for rectification, particularly as a positive fault was later found with the the system later on. Minimum disruption to the pax as there was a spare 767 available at lgw, therefore crew and pax were transferred straight onto the other 767 which the u/s one parked right next too.