Scream of the Shalka (http://www.bbc.co.uk/doctorwho/classic/webcasts/shalka/). Which Paul Cornell wrote the script for; it's technically neither Old nor New Who, since the BBC produced it basically a minute before New Who was launched, starring Richard E. Grant as a Ninth Doctor who was subsequently de-canonized by Eccleston, Derek Jacobi as the Master and Sophie Okenodo as the companion du jour. If you listen carefully, David Tennant plays one of the UNIT soldiers. (He did it for free, being the fanboy he is.) This was meant to be a sequel to the tv movie of doom starring Paul McGann as the Eighth Doctor, but as I said, was overtaken by new tv reality.

Also, I'd say New Who is pretty obvious about the Doctor/Master state of affairs, with the slashiest phonecall ever, and given these two characters, that's saying something, as the Third Doctor and Roger Delgado as the Master started this fine tradition way back in the early 70s.