She has a nice, middle-class, bijou cottage, in a little tasty estate, with its own remote-controlled iron gate, where she lives with her young son, James. Interesting prints of ye ancient Stoke Newington and ye medieval Hackney on the walls. Yes, her home is in the heart of her own constituency, one of the most ethnically mixed and socially deprived in London, but here she is, safely harboured from any urban squalour outside.
And why shouldn't she have a nice place? She's a successful career woman, for goodness sake. At the same time, she goes everywhere on the bus, mixes with her constituents, walks her son to nursery school. She can't drive, just never got round to it. Never been interested in possessions. Took her years to get a washing machine.
She's made some instant coffee, humming to herself. Many of her sentences end lamely in 'Do you know what I mean?'. Tut tut, Diane, after all that privileged education. For a tough feminist, she giggles a lot. Her accent is strong north London. Didn't any of that Cambridge gloss rub off on you? But these are all good signs. So refreshing to meet a politician who looks and acts like a human being.
She was born in Paddington, London, in 1953, coronation year. Her parents had come from Jamaica two years previously - from the same village, but their courtship began in London. 'Guess who's here,' so her father, Reg, was told in the street one day.
He had unloaded bananas in Jamaica, did odd jobs, but in London he sorted himself out, got apprenticed to a welder, and in turn trained his own apprentices. Her mother, Julie, had been a pupil teacher in Jamaica. Here she trained to be a nurse. They moved from Paddington to more affluent, whitey, suburban Harrow, buying their own house. Her dad did pretty well then. 'No, he just worked long hours. Black families couldn't get council houses in those days, so the only way up was to buy. It was only a little terrace house, nothing posh.'
But it meant that she and her brother, Hugh, one year younger, went to white schools. And both did very well. Hugh went to Manchester University and is now a civil engineer.
'I spent my childhood in a book. If I went on an errand, I would walk there and read at the same time. There were no books in our house. I lived my life in the public library. I was small and podgy and wore pebbly glasses. When I passed for the grammar school, I remember handing in my first essay and everyone got a mark except me. At the end of the lesson, I said, 'What's happened, where's my essay?' 'Where did you copy it from,' asked the teacher. She couldn't believe I'd written it. Because I was black. I was expected only to be good at netball - and I was useless. Teachers have entrenched views - and because of that, their expectations of you can mould your performance.'
Not in Diane's case. Nothing seemed to keep her down, even the fact that while she was doing her O-levels her mother left home. She became housekeeper for her dad and brother, till she started having problems with her dad. 'Oh, it was all bluster, really, he kept on saying girls of my age were out working, earning money, why wasn't I? I left home in the end and lived with friends while I did my A-levels.
'The school didn't want me to try Cambridge. In fact, they were hostile. They thought I had ambitions above my station. I wanted to read history, and even the history mistress tried to talk me out of it. I'd got the idea of Cambridge from books, the sort I'd been Hoovering up from the shelves since I'd been a child. That was where people in books had gone.
'I didn't assume I would get in, but, it's true, I did have ambitions far above my station. Many working-class girls did, back in the Seventies. Not like today. Most working-class girls have no expectations. They know there won't be a job at the end, wherever they go.'
She got three As at A-level and waltzed into Newnham, no problem. But before we leave her school days, we'll take a short detour. At sixth-form level, Harrow County, her little state girls' grammar school, had close contacts with the associated boys' grammar. Two of its pupils also went on to be well- known in their own right, one of them a prominent Tory, Michael Portillo.
But he seems so much older. 'We're exactly the same age, but because he's a minister, he looks older. We appeared together in a school production of Murder in the Cathedral. I took evidence from him only yesterday in the House, for a select committee. He's very grand today, with his blow-dry quiff and his Tory grandee manners. I look at him performing and I think he's just another grammar school kid on the make, the child of immigrant parents . . . just like me.'
The other contemporary is Clive Anderson, barrister and TV host. 'I went on his show recently, just because he asked me, and afterwards I said why are you doing this? The only intellectually creative and challenging job in TV is being a director or producer, not walking into the studio and reading the autocue. He never replied.'
Diane was very unhappy in her first year at Cambridge, feeling lonely and isolated. 'It was class rather than race. I was mixing with girls who all had country cottages. The social gap shocked me. My view of the world had been formed by the Daily Mirror. I sat huddled over my gas fire a lot, but rallied in the last year and joined things. No, not the Union. Ghastly place, full of ghastly people. Little did I know that 20 years later I'd be working with those sort of people. . . .'
She got a bottom second. 'I think I peaked at 18.' Her tutor asked what she'd like to do in life and she said 'do good'. So she suggested the Civil Service. Well, university tutors are a bit cut off from the real world. Diane went to the Home Office for two years, then the National Council for Civil Liberties, then decided she'd had enough of doing good. 'It was the first time I'd met liberal do-gooders. I found them very patronising when it came to race issues.'
It was then into the not-so- good-doing world of television for the next five years, as researcher and reporter. By then she was into politics, as a Westminster councillor. In 1987 she decided to stand for Parliament. 'The first and biggest hurdle in my life was getting into Cambridge. Becoming an MP seemed the final hurdle. I'd had three ambitions as a child: to be an MP, have a family, write a book. I've done two so far. Perhaps at 50, I'll retire from politics and write one.'
In the meantime, is she enjoying Parliament? 'It was a bit hard to find my feet. As the first black woman, I wasn't allowed a decent obscurity to make mistakes. Most men can keep their head down and find out slowly how things work.
'It is a very odd place. It's like the Starship Enterprise, circling round in space, conspiring to cut you off from the real world, with its own language, architecture, flunkeys. You can be in there from nine in the morning till midnight. If you're a non-London MP, you can lose all contact with your family.
It can be a dangerous life . . .
'Everyone seemed very elderly and male when I arrived. Now I'm becoming one of the elderly. There are at least a few more women. We have six black MPs, but I'm still the only woman.'
Yes, but you have experienced a female PM. You must have admired her?
'I wouldn't use that word, but she was an interesting study, a woman in a sea of men, completely dominating them. She is loopy, you can see it in her eyeballs, and the men she dominated were all wet. The most amazing period was her defeat. For two weeks, there was an air of suppressed hysteria on the Tory benches. They were like a gang of schoolboys who had dismembered matron.'
Do you think in your six years you've done any good? 'It's my misfortune to be elected in a period of 14 years of Tory rule. When you're in opposition, there's not a lot you can do to change things. Even in government, MPs don't have a lot of power.'
So you regret it? 'No. Being an MP is a good job, the sort of job all working-class parents want for their children - clean, indoors, and no heavy lifting. What could be nicer?'
Good one, Diana. 'No, it is a privilege, and I still see it as that. I enjoy the pastoral work, representing Hackney, and I feel I understand the people here, but the House of Commons is not my club. There's no danger of me being sucked into all that. I look at Roy Jenkins, son of a miner, once a Labour MP, and I see how affected he's become, so snobbish, so silly.'
But surely you yourself must have changed. A politician has to toe the party line, and in the constituency you must not give offence and be nice to bores and prats. 'I do have to be more guarded than I used to be. If I go to a party, I have to make sure I don't have too many drinks or someone will say, 'Ooh, there's our MP, look at her'.'
She'd like to make quite a few changes in Parliament, such as retirement at 60 for MPs, 50 per cent women and a better mix. 'All parties have homogenised MPs. The Tories are estate agents, political advisers and journalists. Labour is full of polytechnic lecturers, plus political advisers and journalists. Oh, and I'd definitely have proper creches.'
Her son, James, is now two years old, born when she was 38. 'It was too difficult to have a child when I was younger, with endless meetings and conferences. I took him in his basket to the House when he was a baby, but now he's at nursery school things are a bit better.'
She was married in 1991 to a Ghanaian architect who already had three children by different women. The marriage broke up after James was born. 'A lot of relationships break down when the woman is pregnant. We are now getting divorced.' Would she marry again? 'I'd have to find a man first. Stability is the vital thing, not necessarily marriage. For most of my adult life, I've been in a relationship. I prefer to live with someone. My parents were divorced, and I didn't have stability. I see it all the time in the House of Commons. At the moment I am living with a man, only he happens to be two years old . . . .'
There's a common view that black men make a habit of fathering children, but not marrying. A racist cliche? 'Of course. You can't generalise. I know enough white men who have left their women. My brother is happily married to his schooldays sweetheart. What you can say, as a generalisation, is that in the West Indies the extended family is very strong. In a rural village, it's known who the fathers of the children are. They don't run away. They're part of the extended family.'
Right, here's another generalisation. You moan about the lack of women MPs, but isn't it because women themselves, even the clever and committed, don't want to do it?
'When I was in hospital having James, the night nursing staff was all black. When I asked the administrators why this was so, they said black women didn't want day work. What rubbish.
'They've been conditioned to say that - ghettoised by men. Same with women and politics. They feel it's not for them, that there's a glass ceiling they'll never get through.'
Perhaps it's also something to do with cynicism, felt by both men and women, towards all political leaders? 'The metropolitan chattering classes have been cynical about politics since the 18th century. Think of those lines by Shelley - 'I met Murder in the way - he had a mask like Castlereagh'. What's happened today is that the mass media has spread this cynicism wider.'
Isn't this a good thing? 'No. What happens when the general public becomes disillusioned with politics is the Tories win. If people think nothing can change, it plays into Tory hands.
'I came into politics to change the world. When I turned 40 a couple of months ago, I looked in the mirror and said, 'So, have you changed the world?' The answer's no, but I'm trying. I'm trying to put something back. We have to keep at it. The gap between rich and poor is growing greater all the time.'
She made some more coffee, this time bringing a large choccy biscuit, just for me, which she looked at, longingly. 'Don't ask my weight. I'm always on a diet, all my life. As an adolescent, Twiggy was the role model, and she haunted me. Now being a bit overweight doesn't worry me so much. It's more important to be fit than worrying about being fat. I want to be healthy for the next 20 years, to see James growing up.'
Ten years ago, as a young lefty councillor, she was quoted in She magazine as saying that the best half-hour in her whole life had been making love to a man in a field. (No name was given, alas, though there were rumours about her and a very, very important media executive.)
Today, her best half-hour would be rather different. 'No doubt about when my all-time best moment was - two years ago, at James's christening in the House of Commons.'
Diane, what an Establishment thing to do. You have become one of Them.
'No, listen, you don't understand. It was an excuse to have all my family in the House of Commons - my extended family, 100 of them. You should have seen the faces of the flunkies and police when all these West Indian bus conductors and railway porters arrived at the House in their best clothes. My relations loved it, and why shouldn't they? It was their chance to enjoy the fact that their cousin is an MP.'
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