Methinks this one is a peregrine, but I've been wrong before. Nearly strangled myself in my camouflage net getting these pictures!
Location: Parque Ambiental, Vilamoura, 21st Feb 2009.
I was watching a marsh harrier approaching when it suddenly took evasive action and I thought it had spotted me. Then this other bird appeared, circled a couple of times, then landed briefly.