Both Mark Kermode and Simon Mayo are Doctors. Mark earned his PhD in English at Manchester University, and in 2005 Simon was awarded an honorary Doctor of Letters by the University of Warwick, not (as Mark would have it) from the University of Made It Up.
Mark has never let Simon forget that within their relationship he is the only one who earned his doctorate, and as such is the only one who should be considered a "real" doctor (though it is doubtful that in any circumstance necessitating someone yelling out "Is there a doctor on-board / in the house?" that Dr K would make himself known). Indeed, Mark believes himself to be twice as clever as Simon, because he laughed twice as much at The Life Aquatic With Steve Zizou.
Apparently Simon got his doctorate just because he was free to attend the ceremony on the relevant day.
Mark first began making regular reference to his doctorate on a day when Simon was due to interview both Lord Winston (who ran late, so Mark filled in) and Sir Alan Parker (who did at least make it punctually). Feeling perhaps a little under-titled in such company, Mark instigated the following conversation:
MARK: Can I just say, if you're referring to him as Sir Alan, can I be Doctor Mark? Since - you know - if we're on titles terms?
SIMON: Ok - as long as you call me Doctor Simon.
MARK: But you're not Doctor Simon. I AM Doctor Mark.
SIR ALAN: What are you, a medical doctor?
MARK: No, I've got a PhD. Of course I'm not a medical doctor!
SIR ALAN: That's not a doctor.
MARK: Excuse me - I have a certificate.
SIR ALAN: But they're two a penny, aren't they, PhDs?
SIMON: Good point, Sir Alan!
MARK: As you were...