Sophie Fiennes, yes, one of that Fiennes family, is late. Her first bus left her stranded in south London because she couldn't squeeze her buggy on board. I'm faintly surprised that a Fiennes has to bus it around – I can't imagine rubbing shoulders with a Hollywood actor like Ralph on the number 78 – but then again, as everyone likes to remind her, Sophie is rather a different Fiennes than her film-star brother.
"I did an interview the other day," she recalls, " and the guy said to me, 'Are you jealous of Ralph? Do you think he's more successful?' And I had to ask him what he meant by successful. 'Do you mean money? How do you measure success?' I mean, Ralph would certainly recognise my success in terms of I'm doing something that I love. I just think it's an ugly question, actually. Because, of course Ralph has a lot more money than me, but that's not the only measure of success; he has masses of female admirers. I don't need them! I'm making films that I'm really challenging myself [with], and I'm going to the maximum place that I can on each film. For me to do that, that's the measure of success."
She is one of two sisters: Sophie is one of seven children raised by Mark and Jini Fiennes, including Ralph, star of Schindler's List and The English Patient, and Joseph, who shot to prominence as Shakespeare in Shakespeare in Love. Then there's her sister Martha, also a film director, the composer Magnus, and Joseph's twin brother Jake, a gamekeeper on the Raveningham Estate in Norfolk. Not forgetting Sophie's foster brother, Michael Emery, an archaeologist, whom her mother took in as an 11-year-old.
In the buggy is Horace, her 10-week-old son. Sadly, he doesn't quite make it past the doors of the Groucho Club, where we meet, but is instead whisked off by her assistant for a snooze in the autumn sun while we chat. It's fair to say his mother is not having the most relaxed of maternity leaves. Popping out her first baby barely three months before her first big-screen feature film premieres was, in hindsight, perhaps not the wisest move, although she later claims it was all planned. She's already learning how such a small person can turn your life upside down.
"Even the fact that getting the bus, with a buggy, I had to wait for two buses because there was already a buggy on board. So suddenly you're slowed down by the fact that you can't get on the bus. That says something about the fact that how, as any woman, as any mother knows, you're juggling."
At 43, she might have waited for the film premiere, but that isn't to detract from the critical acclaim that her feature-length documentary, Over Your Cities Grass Will Grow, has already received. The film, which is about the German installation artist Anselm Kiefer, was among the cream of Cannes, where she was hailed as part of a strong crop of British women. Not that she'd thank me for dwelling on her gender. "I never embark on making films from the point of view that I'm a woman; I'm just making films, and then it's a surprise that I'm a female film director. When I was in Cannes it was like, 'Oh, there's so few female film directors ....' But Over Your Cities Grass Will Grow is an intensely masculine film," she points out.
And she's right, in that the observational documentary is about a man. Kiefer, 65, left his native Germany in 1993 for Barjac, just north of Avignon, in southern France, where he constructs an entire city of towers, tunnels, and bridges out of giant slabs of concrete, broken glass and old lead from the roof of Cologne cathedral. Each building contains one of his artworks, including one hauntingly beautiful giant canvas depicting what looks like a winter forest, with sunlight glinting through the leafless branches. Fiennes's footage, which plays out either in silence or to music by the avant-garde composer Gyorgy Ligeti, captures Kiefer making the forest canvas, by throwing first paint and then ash at the picture before a crane hoists it up, and the ash falls off in huge clouds, leaving behind the painting.
The scenes are mesmerising. At Kiefer's request, Fiennes recorded them for posterity so that Barjac could live on after the artist's departure: he quit his living atelier a few years ago, taking his artwork to Paris but the buildings remain, and, in time, unattended, grass will grow over them. The film, which opens this week, has an apocalyptic feel, an end-of-the-world quality, heightened by that evocative soundtrack.
"It seemed the best way to make a document of it was to make a document of it that could exist in the cinema, because then you could really embrace the scale of what he's made and also the scale of his energy and creative force. There's something quite grand about his creative project even though everything is in the process of being destroyed," she explains. Although it's hardly your typical blockbuster, Fiennes is convinced it will find an audience. "Because it's very visually orientated, I think it's very accessible – once you've put down your triple espresso coffee that makes you think that everything's got to be fast, and quick, and immediate, and you're grabbing your BlackBerry for your next email."
Fiennes, who has the distinctive family eyes and nose, is softly spoken and picks her words carefully, thinking through each sentence. Her ability to re-engage her new mummy brain with work is impressive, given what have to be sleepless nights, for all that she claims Horace is a "very easy" baby. She has clearly taken well to motherhood, for all its challenges.
"I'm really loving it. It's amazing. It re-evaluates your whole sense of being a woman, and women, and what it is to be female. What an amazingly rich thing it is to be female. Pregnancy was amazing. I had this thing where I looked at men and thought, 'Wow, they're empty.' When you see a woman pregnant in the street you don't necessarily think that she's empowered if you haven't had a child, but then everything changes through the experience of doing it. Even giving birth, you can't explain to someone what that really means," she says.
She makes her own birth story, which comes gushing out with barely a prod, sound like one of her observational documentaries. From her seventh-storey window at St Thomas' Hospital, in Westminster, she says: "I had a view of the London Eye, and it was a 25-hour labour, and so I was looking at this view of the river and the London Eye going through from night, midnight, early morning, people going to work, then full day, lunchtime, and now whenever I look at the London Eye, particularly in the evening, my whole memory of birth is locked into that image."
Given Horace's creative family heritage – his father, Michael, is a photographer, and two of his cousins are child actors even though one of them, Hero Fiennes-Tiffin, apparently liked getting the cheque for being the young Lord Voldemort in the sixth Harry Potter film better than actually playing the part – it's hard to imagine him becoming an accountant. But Fiennes is adamant she won't be pushing her son in any particular direction. Unlike her own mother, the novelist who wrote under the name Jennifer Lash, and who died 17 years ago.
"My mother definitely had an agenda for us. We were some kind of creative experiment that she was involved in. So then you spend a lot of the rest of your life trying to extricate yourself from those kinds of debts and expectations."
Horace may not necessarily take the Fiennes surname, which his mother finds "embarrassing". She adds: "I've made this film about Anselm Kiefer's work and suddenly my surname becomes something that overshadows it, so I try to play it down and I try to understand why it's so intriguing." As to the importance of being a Fiennes: "I can't really relate to that, other than that 'celebrity' has value in our culture. And because Ralph and Joseph have both had successful careers as actors and been visible, that success creates a kind of fascination. But I don't know why I would then be interesting on that basis. That doesn't make sense to me."
Perhaps the best way to judge how that Fiennes creativity has rubbed off on the documentary-maker is simply to watch her latest film. It speaks for itself.
1967 Born Sophia Victoria Twisleton-Wykeham-Fiennes on 12 February, in Suffolk, to Mark Fiennes, a farmer turned photographer, and Jini Fiennes, who writes under her maiden name Jennifer Lash. Lives a peripatetic childhood, attending 13 schools and moving frequently between England and Ireland because her father liked to "do up and sell" houses.
1987 After leaving school at 16 and a short stint at Chelsea School of Art, she kicks off her film-making career as a location manager for Peter Greenaway. Works with him for five years on films such as Drowning by Numbers and The Cook, the Thief, His Wife and Her Lover. Meets the dancer Michael Clark who played Caliban in Greenaway's Prospero's Books. Later makes a film about Clark for BBC2.
2001 Awarded a Nesta (National Endowment for Science Technology and the Arts) fellowship.
2003 Heads to Hollywood, not for the glamour of Sunset Boulevard but to make Hoover Street Revival, a feature documentary about a Pentecostal church in South Central Los Angeles, which features the ministry of Noel Jones, the brother of the singer Grace Jones.
2006 Her The Pervert's Guide to Cinema, which she makes with the philosopher and psychoanalyst Slavoj Zizek, is described as "essential viewing for cinephiles".
2010 Over Your Cities Grass Will Grow premieres at Cannes and is released in the UK. Also working on a documentary about Grace Jones, which will take two years to edit.