Had she never been to a garden party? 'No, no and I'm sure I never will. I'm a famous anti-monarchist.'
She walked round the room to a window with less salubrious views of London and mused about the homeless, and the seediness that strikes her more each time she returns. One of her chief regrets about living in Los Angeles with her partner, the film director Taylor Hackford, was that she had given up any involvement in British politics, at least the leftish fringes that actresses seem to glide towards.
There is the hint of a need in her to prove her street cred. She hates the LA tag, preferring to emphasise that she still has a flat in Battersea, and perhaps regrets that people have long forgotten that the 48-year-old star was once a commune- dwelling green - just as it used to be forgotten that she was an accomplished actress with the Royal Shakespeare Company who caused thousands of hearts to beat faster as Cressida in 1968, years before The Cook, The Thief, His Wife And Her Lover or Prime Suspect.
It is the last that brings Helen Mirren back to Britain, the third of Lynda La Plante's taut thrillers in which Mirren's portrayal of the ambitious but vulnerable DCI Jane Tennison won her a clutch of awards. After the first two explored sexism and racism in the police force, the latest, to be shown on ITV on 19 and 20 December, dabbles in homophobia. DCI Tennison is transferred to the Soho vice squad which, with one of those coincidences that enrich sequels, also contains her old adversary, Sergeant Otley, played by Tom Bell.
The passing years have taken little from Helen Mirren's attraction, the blue eyes as piercing as ever, a carefree elegance as disarming as a sexual allure; and they have taken nothing from her good humour, charm and openness. One can make an oblique reference to her past appearances in the gossip columns without being hurled from the top of the Hilton.
''I did do some research for Prime Suspect 3,' she says, 'as I didn't know anything about the vice squad.'
I raised an eyebrow. 'No, I know a lot about vice,' she chuckles, 'but the vice squad and I, our paths haven't crossed, though it was a near thing once or twice.'
She and Lynda La Plante spent time at the vice squad's offices, talking to abused children and young male Aids sufferers, one only 12 years old. They also went to transvestite clubs, where Prime Suspect's writer was startled to discover there was now a notorious red-haired transvestite on the circuit calling himself Lynda La Plante.
For Helen Mirren, the research was the way of getting the details right. 'Saying the right thing when you stop someone on the streets, getting the body language right.
'When I was trying on a costume in the mirror for the first series I folded my arms. One of the policewomen said: 'We never fold our arms. It's a defence action.' They are very into body language, the police. They understand body language very well, much better than actors.
'I can never tell when someone is lying, but they know immediately. It's been quite disarming to spend time with people who spend their lives doing that.
You know you're not hiding a single thing.'
For Prime Suspect 3, Granada TV's head of drama, Sally Head, brought sophisticated London transvestites to the set in Manchester. She felt the club atmosphere could never have been achieved by dressing up extras, who may have been awkward or merely camp.
Though Jane Tennison became something of a cult heroine after the first series for her stand against sexism, Mirren has not totally warmed to the character: 'I never really liked Jane Tennison's brutality or her selfishness, and I don't like her job and could never be involved in a profession like it.
'But I do approve of the way she walks on men and uses them, which is just what men often do to women. I think women are just as capable of that as men. And I think the fact that she could be unlikeable is one of the reasons she was popular.
'Sometimes being a female character is a bit like being a black character. You're treated with kid gloves. You can't be selfish and greedy unless you're like Joan Collins in Dynasty. You can't be an ordinary, flawed person.'
Mirren, who is also shortly to open in a feature film, The Hawk, about a woman who suspects her husband of a series of brutal sex murders, is aware that she has come back to a Britain where violence in film and on television is a hot issue, but does not class Prime Suspect or The Hawk in the nasties category.
'Neither of these films is at all violent. I've no problem with violence if it is the truth and the reality being shown. It's wrong when it's made to look facile and meaningless and you expose children, who have no sense of reality, to a fantasy world of violence.'
While she has some memorable television credits in her back catalogue, including dramas such as Cause Celebre and Dennis Potter's Blue Remembered Hills, it has taken the police genre to bring Helen Mirren to her widest audience yet.
'It's wonderful because it's popular entertainment which hasn't compromised itself in terms of quality and thoughtfulness. You don't have to hurtle between the simplistic and the intellectual.'
And to give her a further sense of inner satisfaction, last year's fracas over an American studio buying the rights to Prime Suspect but wanting to cast a younger American actress in the role may not end in Michelle Pfeiffer or Demi Moore giving a Helen Mirren impersonation after all. The studio, she says, seems to have forgotten about the whole project.
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