I find it easier to believe the journo than George Osborne in this instance... she gave a detailed real-time account whereas he (and Virgin) had the chance and, obviously, the motivation to spin the story later.
I have a bit of sympathy for Andrew Mitchell - yes, he was a twat, but he's not the first person to say something he shouldn't have when tired and angry (ever hear the stories about Gordon Brown on a bad day?), and the police shouldn't have leaked it. Osborne, however, or his aide, was blatantly trying it on.