Combic Book Movies has some quotes by Hugo talking about Vendetta. Yhe article actually quotes from an article in The Hearld Sun, but the important info is that Weaving confirms the reason for Purefoy's depature:

Purefoy "just wasn't working out with him and the directors," Weaving told The Herald Sun. "It was a kind of mutual thing, I think."

This confrims what Eoin found out when he was an extra at the Trafalgar saquare shoot.

If I can find an online link to the Hearld Sun article, I'll post that here.

Thanks to James over at V for Vendetta: Your Views for refering me to this info.