Devious is what you are talking about, its a glorified fan video, that's still unfinished! It's merely worthy on inclusion since the last scene happened to feature Jon Pertwee's final appearance as the Doctor.
Just to fill in a blank, 2 Entertain included Devious as a bonus feature on the DVD of The War Games.
As for the webcast, the BBC website still has it up here:
http://www.bbc.co.uk/doctorwho/classic/webcasts/shada/one/index.shtml
However, I think we've run into one of the wonderful things about the Internet - things become outdated quickly. I have the latest version of Flash and it won't play the old version needed to view. And the other option is Real Player and I don't think anyone's used Real Player in 6 years. You can try the link above, and it might work. A lot of the BBC website material is only viewable within the UK, however I have found that some older items are grandfathered and available worldwide - I was able to watch Shada from this site a couple of years ago and I'm in Canada. But since then I upgraded my computer, and of course today upgrading means losing the ability to do stuff you used to be able to do...
Failing all that, the Shada webcast shows up on YouTube every so often, too...

(Note: I'm just talking about the webcast here - if you want to hear the extended audio drama version, you should order a copy of the CD from Big Finish!)
Alex