'Cause it's the cyclists that need educating...

I'm kinda with Matty here.... just spent far too long on Google and found this.. which seems to be the common patten for Ford rear light clusters on Transits:
http://www.thetoolboxshop.com/0-641-50-durite-commercial-rear-lamp-right-hand-version-1204-p.asp

Which would suggest the top light is the brake light, which would make sense too that the brake light is highest in line of sight to following vehicles. Am going to start tailgating Royal Mail vans as a result to confirm this as an absooooloute.

Royal Mail don't use Ford Transits. That's an LDV Maxxus.
Mind you, that's still the brake light. The indicator is the next one down in the clear section in the middle of the cluster.
http://www.ldv.com/img/content/gallery/van/van19B.jpg