spider-dan said:The spare red is by itself in a sheath with just an earth.
If it's a round sheath (not a flat one, wher the other wire has been cut off and doesn't show) then it's probably to (or from) the switch.
Do you have two-way switching on that lamp - i.e. a switch at each end of the room to control the same lamp?
Have a look behind the switch, What colour wires do you find?
(Lucy runs off to find her screwdriver)