The fame of the 1970 film of The Railway Children persists in the most surprising of quarters. Jenny Agutter was performing (with Daniel Radcliffe) in the West End in the Peter Shaffer play Equus a couple of years ago. “I found it fascinating coming out into Soho every night and all the hookers being there,” she says. “‘Hello Jen, how you doing?’ they’d say, while standing in a corner smoking. They all grew up with The Railway Children, you see…”

Agutter sounds touched at the thought. At 56 she’s completely without airs: as approachable, one guesses, as the 18-year-old girl who played Bobbie all those years ago – and stamped herself indelibly on the British consciousness in the process.

We’ve met because Agutter’s promoting her latest project, a comedy drama starting on July 13 on ITV1 called Monday Monday. It’s about the PAs working in the head office of a struggling supermarket chain named Butterworth’s. Agutter plays Jenny, assistant to the company’s CEO, Roger (Peter Wright), with whom she shares a “dark secret” which threatens to destabilise the office hierarchy. The pair are not the main focus of attention, however, which is reserved for the sex lives of younger staff.

Monday Monday is unlikely single-handedly to reverse the current unhappy critical record of the ITV drama department – but Agutter is enthusiastic about it. “Hopefully Monday Monday is absolutely good commercial comedy,” she says. “Hopefully people will associate with the characters and get drawn in. We’ve left cliffhangers at the end of each episode which have to be resolved…”

Agutter believes too that the programme is an example of how the landscape of TV has shifted recently. “TV is something very different from what it was,” she reflects. “It’s much more personal. Monday Monday is not a sort of Dennis Potter series, analysing something.

“The market has moved away from using TV as an analytical tool,” she goes on. “Because obviously the feeling is now that people relaxing at home don’t want to do that. Drama is less serious, more ‘through the keyhole’.”

I wonder whether she finds it a shame, however, that we have less serious drama. “Yes, I think so,” she concedes. “On TV anyway. There is much more fantastic fringe theatre than there was 20 years ago, though.”

Agutter is lovely to speak to: eager to please, scattering her chat with impersonations of scathing Hollywood types and pushy agents, quoting Bertolt Brecht as smoothly as those Soho prostitutes. After spending 17 years in Hollywood, where she made the films Americans recognise her for, including An American Werewolf in Paris and Logan’s Run, the Somerset-born actress has had a quieter, less glitzy career back home since her marriage to entrepreneur John Tham and the birth of her son Jonathan 18 years ago.

She describes the considerations for her job choices since then as “location, length of time and late nights”. “[The question is] not what I’m going to give up in my life to do a role, it’s how it fits in with my life,” she explains. The result has been many cameos (a part in Stephen Poliakoff‘s film Glorious 39 is up next), radio appearances and talking books – but also the occasional juicy lead. In the TV realm she has played Jane Clark, Alan Clark’s wife, in BBC Four’s 2004 adaptation of his diaries, spy boss Tessa Philips in Spooks and Mother in ITV’s remake of The Railway Children.

Agutter is still happy to talk about The Railway Children and particularly its author, E Nesbit – of whom she is a fan. She is eager to dispel the notion that Nesbit, with her tales of flying carpets and enchanted trees (The Railway Children is one of the more sober stories), can be dismissed as a fantasy writer. As she does so I wonder whether the secret to this unassuming actress’s long popularity is less her brief stint as a Hollywood sex symbol than an attractive childlike quality. The stories to be found in Nesbit’s books are, she says, just as real for children as everyday life is for us. “For me, magic is in your head,” she says. “I don’t know whether I really did or didn’t fly as a child. But as far as I was concerned I flew.”