Thanks Steve.
Fuse is common to all three brake lights and is OK
switch is common to all three brake lights so must be OK,
Ground is common to all lights in the rear cluster and all other lights are working correctly
From your schematic diagram I can't see what the problem could be or where the high-level light wiring splits off. Is there an associated part that explains the wiring colours and connectors - maybe a lose connector somewhere or damaged wire but the diagram doesn't help me with that.