The BBC Last Chance To See website "Radio" page has added an update to say that the 4th episode of the original radio series (Rodrigues Fruitbat) will be available online from November 11th, 2008. This will add to the Aye-Aye and Amazonian Manatee episodes already available. It would appear that the Beeb will be releasing the episodes out of their original broadcast order, as and when Stephen Fry and Mark Carwardine get to the relevant point in their travels to make it relevant.
For the record, below you will find the radio series episode guide as compiled by David Haddock in his excellent article "The Story of Last Chance To See".
The episodes will be "linked" as and when the BBC adds the audio of the programmes to their site. The first entry on the list below is the "chatty ten-minute radio programme of [Douglas Adams] speaking over a background of wildlife sounds" which acted as the precursor to the full Last Chance To See expeditions and radio shows which followed a few years later.
Prelude: Natural Selection: In Search of the Aye-Aye [Aye-Aye Lemurs - 01/11/1985]
1: Ralph, The Fragrant Parrot Of Codfish Island [Kakapo Parrot - 04/10/1989]
2: Gone Fishing! [River Dolphin - 11/10/1989]
3: Animal, Vegetable Or Mineral? [Amazonian Manatee - 18/10/1989]
4: The Answer Is Blowing In The Wind [Rodrigues Fruit Bat - 25/10/1989]
5: A Man-Eating, Evil-Smelling Dragon [Komodo Dragon - 01/11/1989]
6: The Sultan Of Juan Fernandez [Fur Seal - 08/11/1989]