This reaction is all too common for the 53-year-old actress who, despite having a reputation for taking her clothes if the part demands it, has never actively tried to elicit a lustful response. Mirren is bemused, and later tells me that although it's very flattering, there's a danger, "because sooner or later they'll be saying, `Poor old thing, she was very sexy.'" She also adds that it helps never having been really beautiful - she points out that the legs on the Virgin Atlantic ad are not hers. "I've just got by on the good looks stakes," she says. But I have been studying Helen Mirren's face for the past six days, at times sitting close to her for hours on end - in a Jeep driving up dirt tracks, racing along South African highways, enmeshed in conversation during flights - while the rest of the group (a BBC camera crew, radio journalist, photographer and two Oxfam staff) trail in our wake. I would say Mirren does herself down in the good looks stakes. There is definitely a kind of vulnerable beauty, an inquisitive impishness, a glimmer of intrigue. With so little sleep, whole days spent in the blistering heat and living together in such a tight space, none of us look at our best. but Mirren is undoubtedly the brightest and the freshest. She has thought carefully about what to wear, believing that a frock is best as a mark of respect, especially in rural areas. Most days she brings a change of clothes, slipping into them on the back seat of a Jeep.
I am in South Africa with Helen Mirren because she has agreed to use "this peculiar thing called celebrity" to highlight Oxfam's Cut Conflict Campaign. It tackles two issues she feels passionate about, domestic violence - rife in South Africa, with the highest level of rape in the world - and firearms. There are 13 million firearms in circulation, contributing significantly to the high level of instability, premature death and prolonged conflict. Oxfam has been trying for two years to grab Mirren for this purpose. Her fame, and the subject-matter of the Prime Suspect series (crime, violence, rape) make her perfect for the part.
Mirren takes her role seriously. She claims never to have bothered about homework at school but here she is for ever revising. "I'm treating it rather like being on a film set," she explains. "I don't party, I go to bed early and read up on what I have to do the next day." This seems a sensible approach since throughout this six days she will be constantly scrutinised by the camera.
At the press conference, her knowledge and fluency are impressive. She remembers the names of everyone whom we've met and has an intelligent grasp of the issues. She has repeatedly said that she sees herself as an ambassador, or a witness, a conduit for people's stories. In fact, so conscientious is she that she arranged a meeting with Cherie Blair before coming out here. Domestic violence is also one of Cherie's pet subjects.
This is also a fact-finding mission for Mirren. She says that if she were ever to make another Prime Suspect for television - and she admits she now feels more amenable to the idea - she would want it to be about guns. She loves the US - where she spends half her time with husband Taylor Hackford - but its huge flaw is its gun culture, "which has transformed it into a place more brutal and tragic than it need be". During the press conference Mirren breaks down telling of the traumatic moment in the gun- free zone of Mapela when 17-year-old Lydia revealed that her mother had been shot dead in front of her. Mirren describes it as "one of the most powerful experiences of my life". Afterwards, she tells me, "I've been told that our tears aren't useful so I was desperately trying to hold them back." She is amazed when she hears that some local journalists thought she was acting. "If I could turn on the tears as easily as that I'd make a bloody good actress," she says.
Back in Mapela, now gun-free thanks to community action, Mirren had managed to draw out the most reticent of children, working them like a theatre audience. She trained as a teacher but hated it and says she would never have made a good one: "I'd get the giggles and was never stern enough," she explains. But at times on the trip she gets irritated. When one person comments on the "happy, smiling people" we've seen, she disagrees fairly forcibly. "By saying that," she says, "you only belittle their suffering." And when someone asks her if she is enjoying herself, she corrects him straightaway, saying, "enjoy isn't the right word", before adding, "but it's been fantastic."
You don't have to spend long with Mirren to realise she is full of contradictions. Describing herself as "terribly conventional and inhibited", she's also a free spirit - she would have liked to walk off on her own and found the VIP protection squad spooky and unnecessary. She can appear fairly prudish ("Don't be rude," she says to a couple of girls giggling over a naughty word), yet elsewhere can enjoy lewd jokes along with everyone else. She has an air of vulnerability but puts up a guard of invulnerability which is hard to penetrate. I asked her whether she felt separate from the rest of us - after all we were all either Oxfam or press - and she said she didn't, but then later came up and asked me in a concerned way, "I don't really appear to be separate do I?"
She says she likes talking to strangers here because she's actually quite shy. She also admits that "despite being a strong feminist it is lovely to have a man to hide behind". Last year, quite unexpectedly and after a lifetime of claiming she would never be anyone's wife, she married her partner, Hollywood producer Taylor Hackford and claims it was the best thing she's ever done. Initially she was worried it would have a dulling effect on the relationship. "I made him absolutely swear things wouldn't change," she says. "And then it was waking the up the next morning, saying, I'm so excited. I'm so happy. I'm married. This is my husband." She obviously misses him, but she is used to long separations since she divides her life between Battersea and Los Angeles.
Spending a week in such close company with Mirren, intriguing minutiae of her life are revealed. I discover that she is an ace navigator, she rotates between several diets, and she loves shopping. She buys a brolly in Soweto market and I very nearly persuade her to buy a house-coat too - "I love ladies' things," she says. She's also chuffed that her black bag bought from a local store back home, though identical to mine, cost less. She prefers to rough it than stay in posh hotels and can move effortlessly from having a meal in a mud hut in the informal settlement of Bhambayi (one of the poorest, most marginalised communities in the world) to appearing dressed up a few hours later for a satellite link to answer questions about her forthcoming film The Passion of Ayn Rand.
The film is directed by Christopher Menaul, who directed Mirren in the first Prime Suspect in 1990, and traces the tangled 15-year love affair between the late South African author and Nathaniel Branden, her handsome, much younger protege. "That's my profession," Mirren says of her deft switch to the professional actress, fielding questions about her work. "It's all part of the job." During the live show she is, however, at pains to point out where she is talking from. When one critic berates her down the line for the movie's portrayal of an open relationship, she can only laugh, saying: "I don't find that shocking. What I find shocking is the sort of things I've seen here in South Africa."
She is referring in particular to KwamMashu in the northern province of KwaZulu-Natal where we have just been. It is a township in a state of anarchy. Children carry guns into schools, gangs intimidate and murder, and the police station is involved in corruption and gun brokering. Mirren is again much in awe of the women - both black and white - who risk their lives working there with the Programme for Survivors of Violence. It is they who take us into a school which two days earlier saw another terrible atrocity - the murder of a pupil by another pupil for his mobile phone. There is real tension in the air and since we have equipment worth far more than anything ever seen in this area we are all getting a little edgy ... but it doesn't seem to bother Mirren. She is busy chatting to the girls about magazines and make-up and listening intently to the teachers' stories of fear, death and intimidation.
For a moment at the school she becomes upset and irritated. She rails against tabloid journalism after the BBC ask her to do a piece to camera, walking along the trail of blood which goes from the classroom to the outside gate. That afternoon, in Bhamabyi, which is still famous for the atrocities carried out by rival anti-apartheid gangs, she manages to get the partially sighted Cecilia to talk about the night a few years ago when she was raped in her bed, shot in the eye and witnessed the shooting of her young son. That evening Mirren confesses that she is relieved this element of the trip is over: "I hated having to ask Cecilia to repeat her story - there's no catharsis in having a TV camera stuck in your face."
Only once on the trip did we talk about her professional life, though I sensed it was still too recent and raw to ask about critical slamming received last autumn for the production of Anthony and Cleopatra at the National Theatre. Although Mirren actually came off far better than her co-star Alan Rickman, there's no doubt that it was a bruising experience. When she gets home she has a feeling she might be entering into a difficult period of work.
She never expected to make it big in Hollywood: "If you're prepared to get up at five, go to the gym for three hours before doing a day's work and always bring with you an entourage of cooks, chauffeurs, fitness instructors and office staff, then you deserve to make $20m a film," she says. She is irritated when I ask about the endlessly discussed movie version of Prime Suspect. For four years she has been described as too old for the part, but it has also been claimed that she's seen off Meryl Streep and Kim Basinger. None of it is true, she insists, there will be no film version.
I have left these personal questions until our last day. Throughout the trip I'm aware that she does not want to delve into her private life. She's here for Oxfam and refuses to be seen as plugging herself or her next film. After hearing so much about violence, poverty and the terrible destruction caused by arms brokering, she's determined that Helen Mirren the celebrity should, for once, take a back seat
Helen Mirren's visit to South Africa is profiled in `Snapshot', 10.40pm, 8 February, BBC1. To support Oxfam's Cut Conflict Campaign, call 01865 313123