There is the Bianca Jagger of the popular imagination, a stern but glamorous figure who speaks at marches against the Iraq war, and whose accented English may be soft and velvet-burred, but always conveys the impression that she is not to be trifled with. This Bianca gives evidence to US Congressional committees, visits slums in India, documents atrocities in Bosnia, and does not need to warn the new acquaintance that jovial inquiries about riding into Studio 54 on a white horse are not the best of ice-breakers. Her bearing, her gaze, defies you to be so contemptuous as to try to define her by a marriage to a Rolling Stone that ended 25 years ago. Those foolish enough to make the attempt have received monosyllabic replies, implicit in which is a weighty scorn that would bring a deep blush to the most insensitive of interlocutors.
Then there is the Bianca who leans over the table during lunch at Claridge's and wipes some stray Béarnaise sauce from halfway down my chin, an action accomplished with such quiet deftness that embarrassment is entirely avoided; instead, it is a small movement of generosity, of playful intimacy, like a light touch on the arm. This Bianca is delighted by gossip about mutual acquaintances, is quite au fait with their foibles, and readily grasps and empties the rather ridiculous egg-cup-like dish containing my chips after I have found it too hot to handle.
She keeps this Bianca to herself, though, which is why the suggestion that she lacks humour is false but partly understandable, because levity is rarely on show as part of her public persona. It is reserved for her friends, her family, and those in whom she feels she can place a modicum of trust. Having dined with her twice before, I appear to fall into the latter category. When she tells me, at the end of the afternoon, that she does not regard me as being a press person, I know that this is an honour indeed.
Privacy is terribly important to Bianca Jagger because her past personal life has been both a boon, in promoting, and a hindrance, in detracting from, the work that has occupied her for so long. In the pursuit of keeping her personal life out of the public eye, she has developed a technique of switching off that softer side and instantly donning the severe cloak of the campaigner (in which role she has received numerous awards from, among others, the American Civil Liberties Union, Amnesty International, the United Nations and the Rainforest Alliance). She can do it so abruptly that the difference is startlingly marked, as I notice when the talk turns from tittle- tattle to documentaries about the Contras. The smile is sent scurrying from her face to make way for a graver expression. We are talking about much more serious matters, but the lightness vanishes with surprising celerity.
After lunch, when we have driven to her Pimlico residence so she can e-mail an article on Tony Blair's meeting with George Bush, she illustrates the point further. "I've never done an interview in my flat before, ever. Ever, ever, ever," she says, "because I feel that this is my home, my private life. Whatever is public stops there." She points towards the door, but the barrier she refers to goes way beyond a structure of mere wood. I imagine an infra-red screen zapping any unwanted guest who dared to cross the threshold.
On the record - for much of our most amusing conversation must remain, at her insistence, off the record - she finds it far easier talking about the issues that preoccupy her and to which she devotes so much of her life. The amount of time George Bush spends at his ranch in Crawford, Texas, for instance. "CBS had the number of days - 200-plus, I think," she says. (The idea of her taking so much time off is inconceivable.) Referring to the pre-September-11 briefing that has recently come to light, and sent to Bush warning of a possible imminent al-Qa'ida strike, she says: "Wouldn't you have thought he would immediately have ended his holiday to call for an emergency meeting? Now he's trying to find fall-guys - the FBI, the CIA, or the previous administration. It is quite extraordinary that it has taken the US media so long to ask the questions they are asking now - I remember one very distinguished American newspaper even talking about the 'gravitas' of George W Bush immediately after September 11.
"I think that the only hope the United States has today is that he is defeated and John Kerry elected. Another four years would mean he would be able to nominate the replacements for three Supreme Court justices who will be retiring any day now - and the prospect of him nominating justices to fit the vision he has of America is a terrifying thought." Bush, who first had to deal with the wrath of Bianca when he was governor of Texas and she arrived in the Lone Star state as an opponent of the death penalty ("Oh great", he's supposed to have said, "now we've got Bianca Jagger telling us what to do"), is beyond redemption as far as she is concerned. "I don't think that, during my lifetime, there has ever been a president as dangerous as him." More so than Reagan, I ask. "In terms of his polarising views of the world, yes, I do."
Our own dear leader fares little better. "I remember when I first met Tony Blair, when he was in Washington before he was Prime Minister, I was so impressed by his political views and his conviction. But what a great disappointment he has been for those, like me, who thought he was going to be a lightning rod for Great Britain, for Europe, and for the rest of the world. Look who he's in bed with - George Bush. He would like us to believe that he stood shoulder-to-shoulder with Bush out of some grand moral position, and that he believed it was the right thing to do. Well, some people lie and they are conscious that they are lying, others delude themselves that the lies are true - he's one of those."
There's something very total and adamantine about Bianca Jagger's views, a certainty that's enviable as well as admirable. They are not easily-arrived-at opinions aired with an unwanted vehemence, but the result of arduous research, and also of much personal experience, for she has taken it upon herself to be a witness to suffering and injustice. "I think that the understanding you achieve when you see first hand the conditions that people face - people on death row, refugees who are about to be killed by death squads in El Salvador, young girls who have been sold into prostitution - I could read all the books, all the documents and all the reports, but nothing could give me the same understanding as being there, when you are face-to-face with the horrors that some of the people I've worked with have faced."
This awareness of being a witness has been with her ever since her childhood and youth in Nicaragua. "It prepared me for what I do today because, involuntarily, I was a witness, living under the dictatorship of Somoza. I was involved in student demonstrations, and I remember being caught in church with other students when the National Guard was firing tear gas. We'd sought shelter in the church and my father had to come and rescue me."
On one notable occasion in 1981, Bianca Jagger found herself face-to-face with not tear gas, but M16 guns. In Honduras as part of a Congressional fact-finding mission, she was at a UN refugee camp when it was raided by an armed group from El Salvador, who captured 40 men and made for the border. Bianca, her team, relief workers and the captives' families chased after them along a dry river-bed. When they caught up with the captors, the El Salvador death squad turned their guns on them. As the group from the UN camp shouted "you will have to kill us all", it seemed quite likely that the raiders would do just that. After a long silence, however, they turned round, and left the refugees and their would-be rescuers behind and unharmed.
"It was a turning point in my life," she says. "I understood then how important it was to be present when people's lives are at stake. The mere issue of being a foreigner, an American, could help save their lives. When you're faced with situations like that you have to decide whether you just pretend it's not happening or whether you act and do your utmost to save lives. It's nothing to do with bravery." Surely it was, I say. Many of us would like to think we would behave in the same way, but when it came to it we would be too scared. "I'm not saying we were not scared," she laughs. "When you are facing 35 armed members of a death squad, it is very scary!"
Scared or not, she fights on. And one person who ought to be scared right now is her landlord in New York, with whom she is involved in legal action over the growth of toxic mould in her Park Avenue home of 20 years. "I'm based in a suitcase," she says, "especially since my apartment in New York is infested with this mould. I cannot occupy it because it's contaminated, even though all my personal belongings are inside it, including my archive. I am literally homeless in New York."
Redress for her, however, is not on the level of the letter to the local newspaper; she is going to Washington to help a Congressman lobby for legislation protecting tenants. Perhaps because her emotions and passions are so transparent and easily aroused - soup arriving with cream, which she has specifically requested be omitted, is "disastrous" - some may think the mould problem no more than what you or I would call "damp". But housing inspectors have testified that her apartment does contain "B-grade" moulds, which they define as "hazardous to health", and there is no doubt that she is very distressed by the situation, as anyone who has found a home after years moving from country to country would be.
This does nothing to dim her zeal for her forthcoming projects, one of which is a plan to make a series of documentary films with the historian Hywel Williams about the first empires in Latin America, the Mayas, the Incas and the Aztecs, and the malign influence of the US on its southern neighbours in the late 19th and 20th centuries. "The US justified the use of state terror against governments and the people of Latin America because they said they were fighting communism," she says. "But they allied themselves with rogue states and oppressive dictatorships. The mistake is being made again in the Middle East."
While living here, too, she is closer to her daughter, Jade, who calls during our lunch to talk about a party for the jeweller's, Garrard, where she is artistic director, in New York. How do Jade's children see her, I wonder. Do they know what she does, and what do they call her? "I'm not sure if they know," she says, softening again at the mention of her grandchildren. And if ever anyone was under the illusion that Bianca Jagger is a touch too serious, too earnest, here our interview ends with an answer that should dispel that illusion, as she lets the sun in on the warm, humorous person that the public rarely sees: "They call me Bianc-ee," she says, a light, girlish laugh creasing the face of this doughty warrior - who also happens to be a doting, youthful grandmother.