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After leaving university Whittell was writer-in-residence at Nottingham Playhouse (1997).
He is perhaps best known for his play Darwin in Malibu which premiered at Birmingham Repertory Theatre in 2003 in a production directed by John Dove. It was shortlisted for the TMA Award for Best New Play. It was subsequently produced at Hampstead Theatre in London.
His play Happy Valley premiered at the Liverpool Everyman Theatre in 1997 in a production directed by Whittell.
Whittell was then commissioned by Roger Rees to write Villa America about the American socialite couple Gerald and Sara Murphy, and their relationship with Pablo Picasso, F. Scott Fitzgerald and Ernest Hemingway. Villa America premiered at Williamstown Theatre Festival in 2007 in a production directed by Whittell.
Whittell is also the translator of Argentinian playwright-director Rafael Spregelburd’s play Stupidity (La Estupidez). Stupidity was developed at the National Theatre Studio as part of their Channels project. Richard Wolfe directed the English-language premiere for Theatre Conspiracy, in Vancouver, in 2007. 
Other produced plays include Killing Him (Pleasance Edinburgh), and Party Tricks (Nottingham Playhouse) directed by Martin Clunes.
In 2010 Whittell was commissioned to write a new version of A Christmas Carol for the Guthrie Theater in Minneapolis. It opened in November 2010 in a production directed by Joe Dowling with Daniel Gerroll as Scrooge. The production has been remounted every year since.
- Killing Her (Old Red Lion)
- Killing Him (Pleasance, Edinburgh)
- Party Tricks (Nottingham Playhouse)
- Happy Valley (Liverpool Everyman)
- Villa America (Williamstown Theatre Festival)
- Darwin in Malibu (Birmingham Repertory Theatre)
- Stupidity/La Estupidez (Theatre Conspiracy, Vancouver)
- Clever Dick (Hampstead Theatre)