Thanks for your comments!
Beyond the military circunstances, we can wonder if other factors did influenced D'Urban's decision. Namely, "that" human factor, political factors, emulation ...
1. D'Urban was one of the highest rank british officer in portuguese service, a close friend of Beresford. He was named for the important task of commanding the advance guard of the allied army. Can we see "that" political factor in is nomination and in D'Urban action in Majadahonda (unconsciously or maybe not...)?
2. What about competitive spirit between british officers in portuguese service and those serving in the british army ? There's any references on this?
Too much psychology ?! :)