As awesome as this episode is, there's a pretty big continuity error. The Master long ago used up all his regenerations (a Time Lord can only regenerate 12 times). In fact, that was the major motivation for his dastardly plans in both 1976's "The Deadly Assassin" and 1981's "The Keeper of Traken". He was nothing but a walking corpse, kept alive by sheer willpower. At the end of "Traken", the Master was able to use what was left of the Keeper's power to take over Tremas' body, thus giving him a new one to inhabit. It's true that in 1983's "The Five Doctors", the High Council of Time Lords offered the Master a whole new regeneration cycle if he rescued the Doctor, but since he betrayed them all I highly doubt they gave it to him. Even in that crazy 1996 Fox TV movie, the Master couldn't regenerate anymore and had to take over Eric Roberts' body (presumably with some residue of the Keeper's power, though that movie is such a mess it's never quite clear what's going on -- at one point the Master appears as a glutinous CGI serpent!).
So how the hell did the Master regenerate in "Utopia"?