A script for an episode of Dr. Who written by the late Douglas Adams,
author of the bestselling Hitchiker's Guide Trilogy, will finally be produced and
seen next year as a BBCi webcast. Filming originally began in 1979 on the episode, "Shada", which apparently was intended to conclude the seventeenth season. Paul McGann, who played the Eighth Doctor in the 1997 Fox movie, will star, and K9 and Romana are the companions.

the BBC news article

Big Finish Productions, home of The New Audio Adventures, who will be producing it They reprint the entire BBC press release, which gives a lot more detail about the cast.