In a private members’ club in Soho on a sunny day, Billie Piper and I are discussing the burning issue of the day. Indeed, the issue couldn’t be more burning. It is cystitis.

“And you get some women going: ‘Oh, I think I’ve got cystitis, I might have some cranberry juice’,” Piper says, in the tinkly, light-hearted voice of a woman with faux-cystitis. “But that’s not cystitis! Cystitis is when you’re crying and writhing around and you can’t leave the house, and you are going, ‘Why, why have I got this?’, and you’re in absolute agony. I’ve had to cancel loads of work with cystitis. I’ve had to cancel loads of my life with cystitis.”

With George III it was porphyria. With Frida Kahlo it was cerebral palsy. For Billie Piper it is what those of us who suffer from it refer to as the Real Big C. She has an Achilles’ urinary tract.

Piper’s decade-long battle with cystitis is one of the narrative mainstays of her thoroughly entertaining recent autobiography, Growing Pains. Recounting the first month of her infamous relationship with the DJ Chris Evans, she muses on how the general public would have presumed a bunk-up between an 18-year-old pop-sy and her millionaire media lover to have been a debauched, frenzied sex-fest.

The reality, as she explains, was very different. Having run away to a five-star hotel in Madeira, the reason they never left the hotel room was, in fact, because she had recurrent, haemorrhaging cystitis, and could manage only a “few acid drops” at a time. “It wasn’t exactly love’s young dream,” she says, drily.

It was at about this time in Piper’s life — 2000 — that I first felt the developing of a spiritual kinship between her and me. Tabloid reports at the time hinted darkly that Piper had a drug problem, and when she collapsed in a nightclub and was taken to hospital with “a kidney infection”, the general media response was a sneering “Yeah, right. Kidney infection. Sure.”

However, I read those reports and thought, “I’ve had cystitis so bad I could have collapsed in a nightclub! I believe you, Piper!”

“I can see why it looked dodgy,” Piper admits, dragging on a ciggie. “I mean, you spend the whole night in the toilets, of course everyone thinks you’re going to be doing coke. But you’re like, ‘No, I’m pissing razor blades, not chopping out lines with them’.” While there are many ailments that have a certain dark glamour to them — depression, bipolarity, heroin addiction, syphilis — excessively alkaline urine certainly isn’t one of them. The fact that Piper is so wryly willing to trade in a perceived rakish edge to her pop-moppet years — cocaine overdose — for a vision of the truth — sitting on a lavatory and crying — is a very telling detail.

“Being honest is probably the only way to cope with the whole fame thing,” she says, leaning back in her chair. “You have to be as honest as you can about your life, while not dragging in or ruining the lives of those around you. Hypocrisy makes you a target.”

Her voice is much posher than one expects — a hang-over from the upper-middle-class role she is currently playing in the West End, in Christopher Hampton’s Treats. She wears glasses, and looks wiry and tired, but also spends the whole inter- view being unexpectedly candid, cracking jokes, roaring with laughter, and using a pleasingly dated lexicon (“converse”, “pins”, “nuts”, “notion”, “horrid”) that points to a great deal of her education having taken place during all-day drinking sessions with older men.

In a market crowded with celebrities, Piper cuts a singular swath. A teenage pop star at the age of 15, she was unusual from the start. While most teenage chanteuses are chosen for either cuteness or incipient Lolitadom (see Aguilera, C. and Spears, B.), Piper was an altogether different kettle of fish. With her strong brow, ample mouth and gleaming teeth, she looked like some feral Wolf Queen, just as likely to eat her fans as sign their autograph books.

Her lupine charisma lent her first hit single, Because We Want To, an unexpectedly threatening edge. While the lyrics ran: “Why you gotta play that song so loud?/ Because we want to!/ Because we want to!”, they could just as easily have gone: “Why you gotta worship the wolf-god, Fenris, and pray for the beginning of Ragnarök?/ Because we want to!/ Because we want to!” In a world of anaemic mopsy jail-bait, it all felt like a rather glorious mistake.

To Piper, it felt like a nonglorious mistake. Nearly three years later — burnt out by a crippling promotional schedule and still unconvinced by pop material that she found “at heart, not good enough” — she effectively resigned from her own career. She did this by the unique expedient of embarking on a two-year holiday with Chris Evans, whom she met when she appeared as a guest on TFI Friday.

Theirs was an unlikely but idyllic-sounding courtship and marriage — learning about speciality cheeses, playing Victorian parlour games, drinking vintage wines, jumping on private jets to look at the Coliseum, and spending hours digging in the garden of their Cotswolds house.

It was in the four restorative years that she spent with Evans that Piper started to assess what she wanted to be. Not a one-woman adolescent Spice Girls, as her record company had envisioned her. Instead, she realised that she was, essentially, an earthy, domesticated autodidact, who wanted to pursue — no matter how unlikely it seemed — an acting career.

And so here we are, three long years later. Piper’s simultaneous blend of ballsiness and vulnerability — all served up in that fabulously extreme face — has resulted in her being one of the most employable actresses of her generation. After Doctor Who — an inspired, unexpected piece of casting that resulted in Piper becoming one of the all-time great Doctor’s assistants — she has the pick of British drama projects. Her forthcoming lead role in ITV1’s Mansfield Park — her Fanny Price is a sunny, spirited child with a musky edge — is her second major role in three months, after playing Sally Luckhart in the BBC’s adaptation of Philip Pullman’s Ruby in the Dust . Yet when, after just two series, she unexpectedly walked out of the role of Rose Tyler in Doctor Who — “I knew I’d get complacent if I stayed” — she had nothing lined up.

“I had no idea of what I was going to do. And I like that,” she says, with a lascivious flash of her enamel. “I like scaring myself. I never want to be bored.”

However, this is a statement she almost retracts moments later, on considering her latest project — a revival of Christopher Hampton’s Treats , which transfers to the West End this week. It’s her debut on the stage, and Piper is under no illusions as to whose fault it is that she’s doing the play.

“David bloody Tennant,” she says, naming her co-star in Doctor Who . “He’s a massive theatre nut. He would speak so endlessly about it, and how good the feeling was.”

Piper had thought that “being in the theatre” would mainly involve “turning up to work in glasses and a tatty scarf and smoking lots of rollies”.

“However, when you start doing it, you realise you do actually have to work at some point. It’s making me very anxious. I think about it on the toilet and in the bath. I am regretting it! But I want to learn as much as possible about this profession, and everyone says the stage is the best way.”

In Treats Piper plays Ann — “basically a nag” — who is compelled to revisit an abusive relationship. Hampton designed the piece as a study of a woman’s self-destructive edge, and Piper agrees.

“Women are incredibly strong, but often the only time they’re weak is when they’re with a dysfunctional man,” she notes, pithily. She adds that she has “experience of that”, but clearly isn’t referring to her current relationship with her Treats co-star Laurence Fox, son of James Fox. “I love being on stage with two big strapping lads,” she says — Fox is 6ft 3in and Kris Marshall, her other co-star, is 6ft 2in. “They make me feel . . . delicate.”

Piper’s best scene in the play is completely wordless. It is five minutes long, two of which she spends watching television and three crying. It is an almost rhapsodic display of sobbing — snotting, pained, slightly writhing, but all desperately repressed. There’s nothing showy about it at all.

“I think I like TV acting because it’s all small and contained and mumbly,” she confirms. “I find that easier.”

Quiz her on her politics and she admits to being confused and angry, and spending dinner parties “shouting, and feeling frustrated that things don’t change”. She claims to think about global warming “about 12 times a day at the moment” and is on the verge of buying a hybrid car, but she points out ruefully that, if she did, “that would be the end of the Porsche” — a 1989 Carrera, white with a fin. Looking at the car on her driveway makes her happy. “I’m quite simple,” she says.

Perhaps her most surprising claim is that she’s not a feminist. “I like cooking for a man, and cleaning,” she says. “I like women to be women, and men to be men.”

Obviously no one has ever pointed out to her that a woman who takes control of her own career at 19, holds her own in an industry where most people are twice her age, and, most pertinently and admirably of all, declines any alimony claim when divorcing her millionaire husband (her marriage to Evans was annulled, wholly amicably, in September) is a feminist through and through.

I, however, am not going to be the one to tell her. Instead, I’m too busy giving her a bottle of potassium citrate, £1.86 — something which, after years of experimentation and suffering, I have found to be the finest cystitis remedy known to womankind.

“Oh my God!” Piper yelps. “Wow! This is amazing! Why have I never heard of this? Thank you! I feel like crying!”

And, in many ways, who is to say that this gift is not the equal of emancipation and gender equality?

Treats previews at the Garrick Theatre, W1 (0870 8901101), from tomorrow and opens on Feb 28. Mansfield Park is on ITV1 in March.

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