Peter Firth, who plays Spooks boss Harry Pearce, has revealed what he thinks makes Spooks so successful as series nine of the hit spy drama is released on DVD.

Speaking exclusively to whatsontv.co.uk, Peter said that Spooks has enjoyed such a long run because it has learned from its mistakes.

He said: “It’s a process of trial and error. Now we’re in our tenth year, if we haven’t got it right by now then something is sadly wrong. Over the years, the show has got better and better and that’s very rare in television.”

He admitted that he knows nothing about Series 10, due to be shown on BBC1 later this year, and is always kept very much in the dark.

He said: “Scripts usually arrive the day before production is due to star and sometimes not even that. Sometimes we just get a few pages.”

The ninth series of Spooks which saw the agents, including newcomers Sophia Myles, Max Brown, Simon Russell Beale and Laila Rouass tackle online hackers, Somalian terrorists and an assassination attempt, is available to buy now on DVD.

CLICK below to watch our interview with Peter Firth