How do you change the brake light bulb on a fiesta 54 plate?

This, I presume is the last year of the type which had the squarish design of lights and rectangular reflectors, before Ford changed to the round lights and reflectors? If this is the case, there's a big plastic wing nut holding the light cluster in place and it's not obvious! You have to reach inside to the back of the light cluster and feel for the wing nut. Unscrew it and the light cluster will come loose. Then get hold of the light cluster with both hands and pull forward firmly, to unplug the two plastic pegs on the outside. If it's the newer design with two screws on the edge covered by the hatchback, they are T25 Torx screws, not Phillips or Pozidrive! The bulb is the usual bayonet 5W/21W side/brake light bulb.