Watership Down lies in Hampshire, England and describes not one, but rather several hills. 1972, Richard Adams writes the self-titled book and let a rabbits tribe travel to there. A large area of the Down is privately owned by Andrew Lloyd Webber, who did also an arrangement for the series.
The rabbits, lead by Hazel, are displaced from their old warren and search now a new home. But their freedom is threatened by the antagonize tribe Efrafa, lead by General Woundwort.
6 years later, the story was filmed by Martin Rosen with the music from Mike Batt. The title song "Bright Eyes" is sung by Art Garfunkel. A sequel of the book hit the market in 1996 and three years later, the tv series was produced, with what everything around here deals.
It was produced again by Martin Rosen. Mike Batt did the music again for the series. The main theme stays the same, but it was covered by Stephen Gately. The arrangement "In Fields of Sun", that you can hear in the first two seasons of the series, was dome by Andrew Lloyd Webber. Famous voice actors like John Hurt, Richard Briers (both voice actors of the movie, too) or Kiefer Sutherland lend the rabbits their voices.
The first two seasons are new interpretations of the books. The third season is a sequel. It starts with the destruction of Efrafa and deals later with Darkhaven, Woundwort's birth place. Destroyed by men, it is colonized again by rabbits and they wait for Woundwort's return. He will lead his army to Watership Down into the last battle. In addiction, the creators of the series are using supernatural powers. That departs the series even more from the books.
The drawing style changed also in the third season. For clarification, you see subsequently a graphic of Bigwig and how he changed from 1978 to 2001.
There are rumors, that this is due to copyright-contentions. But that can be excluded, since the credits of all three seasons say the same:
"Character Design and Development Copyright Funbag Animation Studios Inc. and Telemagination"