“Put your left hand on my bum,” demands Strictly Come Dancing’s head judge Shirley Ballas. “Now stay still and I’ll do the work.” Small but powerful, graceful and effervescent – even in an aircraft hangar of a studio – the 57-year-old from Wallasey is an intensely physical presence who twirls around me in a flash of movement, sparkles and very firm instructions. “Keep your feet pointing out, darling,” she says. “That’s it!”

I’m not entirely sure which dance move this is, but I try to keep my size 12s from crushing her manicured toes as the gold lamé-clad, pint-sized sensation shouts out explanations of what we are doing. At the same time, she demonstrates the lithe skill that won her two international Latin American dance championships (in 1983 and 1995) and the professional sobriquet Queen of Latin, before she gave up competing to train other dancers.

Her habit of using technical terms to describe what’s going on behind the drama and glitz has been a large part of her success as a replacement for former head judge Len Goodman, though she has tempered it as the series has progressed. “I just want the people in the audience to go, ‘Oh, there’s a chassé,’” she says as we pause. “‘Or, ‘Oh, no! She’s got no rotation in her hip.’ I want them to actually spot it, too.”

It was Ballas’s 81-year-old mother, Audrey, who told her to change her style after the first two weeks. “She said, ‘I love it that you’re giving your technical advice, but what are you talking about? Can you put it a little bit more in layman’s terms?” It worked, and Ballas is now as established on the judging panel as Craig Revel Horwood, Darcey Bussell and Bruno Tonioli.

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Ballas is immensely enthusiastic about her fellow judges. “I’ve known Bruno for 11 years and he’s as mad as a hatter,” she says. “Craig is a doll. I just love him; I’d like to marry him. He’s everything that you would want in a man: he’s kind, he’s caring, he’s thoughtful – he’s just a wonderful human being.”

And Darcey? Ballas sighs. “She’s like Snow White, she’s so beautiful. Our backgrounds are worlds apart. I come from a housing estate, the underprivileged if you like, and she comes from privilege. But her persona and her lovely attitude just work well with mine. She’s so kind I want to cuddle her all the time and give her a squeeze.”

It’s tough to shine alongside Bussell, but Ballas works hard to pull it off. “When I was a young girl, I had some weight issues,” she says. “I went up and down when I first got married.” Today she avoids alcohol and enjoys “Bikram yoga in a room heated to over 100 degrees”.

Before shows, Ballas FaceTimes Audrey so she can give her the once-over. “She’ll go: ‘Yes, like the hair. No, don’t like the lipstick’.” Then, after every show, Ballas watches it back to see what she could do better. “You’ve got to walk across the floor and arrive on time,” she says. “You’ve got to sit without slouching and keep your composure. I’m an emotional person. When Simon Rimmer did that waltz [to You’ll Never Walk Alone, in honour of the victims of Hillsborough], I got lost in a lot of memories. It was so moving for me, I lost all sense of time. And I still don’t quite get that scoring paddle in the right place – it’s covering my teeth! I have to practise that because I’m a perfectionist.”

The scoring paddles are lined up in order, 1 to 10, under the judges’ desk. There is no zero paddle (“You can’t give a zero paddle to Craig”). She’s yet to drop one, but in Blackpool, when the judges made their entrance, Ballas did arrive at her mark a few seconds after Bruno, Craig and Darcey. “Cut me some slack!” she says. “I came out and my mum was sat right there in the crowd and she likes to see the back of my dress. How does it look, has it got a tail on? I had these great big heels, so as I turned I couldn’t move, I was mincing. So, yes, I was late across the floor, but I had to blow my mother a kiss.”

Audrey still lives on a housing estate in Merseyside, though she can enjoy family holidays in California now, thanks to the success of Shirley – who in replacing Goodman slips into a £200,000 salary space – and her grandson Mark, a professional dancer on the US version of Strictly, Dancing with the Stars. It wasn’t always so. “When I was young we didn’t have a pot to piddle in,” Ballas says. “Our father left when I was two and my brother David was three. Other aspiring dancers had lots of private lessons a week, I had maybe one a month. I practised in my bedroom. I taught the other kids to cha-cha-cha in the schoolyard. I was always the judge, but I was practising my own skills. And we couldn’t afford to go for a holiday.”

The first time Ballas crossed the Mersey and travelled up the Lancashire coast to Blackpool, it was on a coach, aged ten, as a competitive dancer: “I danced in the Tower Ballroom until I was 16, and then the Winter Gardens.” She went on to become a champion in a world that can be unforgiving. “You have to have a strong spine in this business,” she says. And is hers? “Strong? I have an iron spine. I come from the north of England, I come from a tough mummy.”

The ballroom dancefloor both formalises and idealises the relationship between men and women. In reality, Ballas has had a messy time with the opposite sex. As well as divorces from two husbands who were both dance partners (Sammy Stopford and Corky Ballas), she has suffered the tragedy of her brother David killing himself 14 years ago and her father leaving when she was very young. “My dad wasn’t really part of my life at all growing up, until David died, when we grew closer,” she says. “I love him to bits. And we never talk about it, but I like to think that somewhere in his heart, I’m Daddy’s girl. I’d like to have that feeling. He’s very proud. Strictly brings families together, and it’s helped bring my family together. My mum’s sister and my dad’s sister, my dad and some old friends that maybe we’d lost contact with have reached out. It’s been fantastic.”

When Ballas joined Strictly, the BBC, worried about press intrusion into her private life, gave her a social media team to help her with her Twitter account. “I was warned not to read it,” she says. “Which is like saying to a child, ‘Don’t put your finger in that light socket’. I’m curious! But I don’t know how some of the people say what they do. To say they hate me? Hate is such a strong word; my child doesn’t use that word. When they write those terrible things on Twitter, I try to take a step back, take a breath and think: ‘What situation are they in?’ They’re judging me, but I don’t have to judge them.”

She does have to judge the dancers, though, and we don’t always agree with her. “You mean Aston, don’t you?” she says. Aston Merrygold’s departure, after losing a dance-off with Mollie King – who, in all fairness, is not half the dancer Merrygold is – was one of the shocks of the series. “Aston was an absolute natural dancer, wasn’t he?” Ballas agrees. “There was nothing that that boy couldn’t do and he shouldn’t have been in the dance-off. But whose fault was that? It certainly wasn’t mine. It was the public. Should Jonnie have been in the bottom two? Should Debbie? I have to do my job once you vote. I have to do my job based on what the country has decided.”

The tabloids were aghast, but it was lost on Ballas, who doesn’t read the papers any more. Instead, the Strictly publicist sends her a text if there is news. Her second husband (from 1984 till they parted in 2002), and father of Mark, the American Corky Ballas, recently gave the tabloids a lurid account of their marriage, including the accusation that she had taken many lovers and ignored him for weeks. She says his version was like “being thrown under a bus”, but is otherwise remarkably sanguine about it. “He’s the father of my son, and I love him,” she says. “You’ve got to forgive, haven’t you?”

Dancing is a close family world. Ask Ballas to pick her dream celebrity guest for next year’s Strictly and she names the ex-girlfriend of her onetime ward, and regular Dancing with the Stars professional, Derek Hough. “I think Cheryl Cole would be amazing.” But what about Ballas – has she committed to the next series herself? “If I did,” she says, looking at the BBC press officer who has joined us, “It would be, ‘Shirley, keep your mouth shut.’”

What about another controversy, the unfairness of some dancers, such as Debbie McGee, having had previous experience? “I think Debbie’s done a little ballet, but who hasn’t done a little ballet or a little tap? Who didn’t do a little hip-hop? They’ve still got to learn the Latin and ballroom. Now if you find me somebody that’s completely trained in Latin and ballroom, who has won every championship, then I might look and go, ‘Oh, I’m not sure about that’.”

She must, I suggest, have a good idea who is going to win now? “Just when I think it might be this one,” she says, “then it’s that one, and then it’s another one. Look at Gemma in Blackpool. She came out and kicked ass!”

Now we’re no longer in a clinch, I can safely ask the question that occurs to everyone who watches couples, like Mollie and AJ, developing intense relationships over the weeks – do good dancers need to be attracted to each other?

“There has to be chemistry,” she says. “But chemistry is chemistry. It doesn’t mean that you’ve got to marry them or fall in love with them. You just need to have some sort of spark.”

Ballas was only 17 when she moved in with her dance partner Sammy Stopford (whom she then married). “We became the Non-stop Stopfords. Today the world champions aren’t married, they have separate partners, but that’s what you did back then.” Does she regret that? “I don’t regret any single thing in my life.”

The real romance of Strictly, Ballas says, is with the atmosphere, the pizzazz, and the life-changing thrill of dancing live in front of millions. “Look at Susan [Calman], who never used to wear make-up,” Ballas says. “She comes out every week and she is transformed.

“It’s not just about the dancing. It’s about how they walk, how they talk, how they dress, how they put themselves across. Look at her partner Kevin Clifton; he waited five years to do that paso doble. I’ve never seen anybody so excited. His dream came true. That’s what this show is about, making dreams come true.”

Photograph by Nicky Johnston

The Strictly Come Dancing semi-final is on Saturday 9th December at 6.45pm on BBC1