Ever since we saw the Valeyard in Trial of a Timelord, the very idea of him has been an interesting one, a darker version of the Doctor.

With Chris Ecclestones era, and becoming more and more so under David Tennant, there is a darker side that is slowly bubbling up in the performances. As with what Donna says, "sometimes you need someone to stop you". And in the wonderful Human Nature/Family of Blood, we got a vision of what an avenging Doctor could look like, and why indeed he's the stuff monsters have nightmares about.

The new "Half Doctor" who surfaced within Journeys End seems that step darker still. More in touch with his emotions, capable of telling Rose what the real Doctor couldn't - but what about his anger?

I don't think we'll ever see the Valeyard in the New Series, as it's not really as mainstream a character as say the Master or Davros.

But I think we saw a tantelising fan morsel that the Half Doctor could easily turn into the Valeyard or a Valeyard-like nemesis if Rose fails to sort him out (even if it doesn't quite match what was said in Trial).