We’re not talking about buying Vogue here, or even visiting their offices. We’re not talking about Isabelle being in Vogue (which she is, of course, on a regular basis). We’re talking about Jenny being in the June issue. And on it. On the cover. And as far as Jenny’s concerned, it’s all my fault.

It’s not actually my fault – it’s Crow’s. Two Christmases ago, Crow launched her first high-street collection for Miss Teen. Just a few pieces. Jewel-coloured party dresses, skirts and crystal embroidered tee-shirts, mostly. Finally it meant that thousands of girls could wear her stuff, instead of just a lucky few. The collection sold out almost overnight. So naturally, they asked her to design another one. This time it’s a summer one – all white cotton, layering and clever cutting. It’ll be launched in May, but they’re already busy on the publicity for it.

Making Jenny the face of the new collection was, admittedly, my idea. Jenny is curvy and gorgeous and looks fabulous in Crow’s clothes. When the news first started to leak out about how good this collection was going to be, the editor of British Vogue decided to stick her neck out and put Jenny on the cover in one of Crow’s new designs, despite the fact that she’s a size 14 and isn’t exactly super-famous, having been in one school musical, one film and one play. At first Jenny was thrilled, but the closer the shoot gets, the more she’s changing her mind about me being a fashion genius and deciding that I am, instead, DELUDED AND CRUEL.

But that’s just nerves, as I keep telling her. She looks great, in her redheaded, bouncy, bubbly way. And even if she gets the zit outbreak to end all zit outbreaks, they can just airbrush it out. All she has to do is smile. She’s an actress. Can’t be that difficult.

You’d think.

Jenny’s seventeenth birthday is coming up soon and in the past few years she has learned many things. One is that you can’t trust a Hollywood Teenage Sex God as far as you can throw him. Another is that yellow trouser suits (Tokyo premiere for the movie three years ago) are a no-no. And finally, if you’re not super-confident about the way you look, don’t get your best friend to sign you up for the cover of Vogue. But if it’s too late to back out, blame her and look at her accusingly whenever you get the chance.

When the day of the photo shoot comes, Jenny sits in front of the wall-size mirror at the studio, having her newly-coloured hair done and looking like she’s about to be shot by a firing squad, instead of Ted Regent – otherwise known as ‘the new David Bailey’ or ‘the man who makes cool hot’. The expression in her eyes flits between terror, whenever she catches sight of herself in her cover-ready makeup, and fury, whenever she catches sight of me, bobbing around behind her.

They’re going for a sort of ‘visiting alien’ look. Jenny’s wearing super-pale foundation (unnecessary, given how white her face looks already), loads of multicoloured eyeshadow, feathery false eyelashes in peacock colours and silver lipstick. She’ll be almost unrecognisable, I assure her.

‘They’ll recognise my boobs though,’ she says miserably. ‘Even under all the clothes. And my fat shoulders.’

‘They’re not fat. They’re curvy.’ How often do I have to tell this girl?

‘And my fat arms.’

I sigh and give up. Luckily the hairdresser takes over. He seems to be used to reassuring nervous models before a big shoot.

‘You’ve got gorgeous, delicate wrists. I’ve been admiring. And your hair is to die for, girl! The shade! That colour’s taken like a total dream. Everyone will want Jenny Burnt Orange by the time you’re done. E-ver-y-one. Trust me.’

Eventually, when hair and makeup and wardrobe and nails and the fashion editor from Vogue and Crow and Amanda Elat, who runs Miss Teen, are satisfied, Jenny shuffles out to pose for Ted Regent. We’re in a studio in Shoreditch that used to be a workshop or a storage area, I’m guessing. All white-painted brick, with a glitter ball in the centre and a large white background for Jenny to pose against.

‘I could picture doing a show here,’ Crow says happily.

I agree. It would be perfect for a little catwalk show. Atmospheric and exciting, especially with Lady Gaga playing at full volume on the sound system, like she is now. I imagine where I’d seat the photographers and the audience, and how I’d organise the models coming out and what I’d do with the cute little gallery upstairs . . .

In front of the white background, Ted Regent is arranging Jenny on a chair. He looks like a model himself – all skinny jeans and designer stubble. He seems to have the energy of a hyperactive four-year-old. One minute he’s on his knees, adjusting Jenny’s ankle. Then he’s back on his feet, changing the angle of her head. Then he’s dancing around, calling out emotions for Jenny to act and singing along to snatches of Poker Face. Jenny, on the other hand, looks like she’s taken the song literally: her features are set rigid and it’s impossible to tell what’s going on behind her eyes. I bet it isn’t like this when Isabelle’s in front of the lights.

There’s a break, while everyone who can clusters round a laptop to look at what they’ve done so far. They tweak the clothes, try another few poses, cluster round the computer again and then Jenny’s taken off to put on outfit number two. Meanwhile, Crow and I stare at each other. She’s done a beautiful job with this collection and personally I can’t wait to wear it. This is the moment when it should all come to life. It’s about to get the biggest splash of publicity we could ever imagine. But so far, our model looks more like something out of a wanted poster. My mistake, of course, but Crow’s too nice to say so out loud. Instead, she gives me a wonky smile.

Jenny comes back in a tunic and leggings, accessorised with a big scarf and chunky jewellery. She tries out a few more poses. Increasingly, Ted gets her to turn away from the camera and shoots the back of her head. Understandably. It’s the most animated part of her.

It’s early evening before we get to go home.

‘God, I’m glad that’s over!’ Jenny says, slumping into a taxi beside us in jeans and a puffa jacket. Oh, and still wearing the feathery eyelashes. She wants to show them to Gloria before she takes them off. Then she’s going to give them to Stella, the cat, as a present. Stella will love chasing and killing them.

She rootles through her handbag for her phone, to tell Gloria she’s on her way. When she pulls it out, she looks surprised.

‘Ooh, a text. From Jackson Ward. You know, the composer.’ She frowns. ‘Oh, he sent it ages ago. Damn.’

She checks her watch. But as I vividly remember from her call, New York is five hours behind. Although it’s quite late in London, it’s still a perfectly reasonable time of day over there. She calls him back.

‘Hi! Jackson? It’s Jenny Merritt. You called me?’

And from that moment on, her face gradually transforms. By the end of the call, she’s glowing. If Ted Regent could have got that out of her for even a couple of frames, he’d be going home a happy man.

‘What is it?’ Crow asks, as Jenny stuffs her phone back into her bag.

‘Jackson wants me,’ she says. ‘A couple of the producers who saw the first workshop are interested in staging the show. They want to do another workshop and make some tweaks.’

‘With you? Again?’ I say, to be sure.

Jenny gives me a hurt look for ever doubting her. ‘Jackson says, as far as he’s concerned, he’s found his princess. Bill agrees. I just have to convince these producers. Jackson said, “You’re my Elizabeth, Jenny.” Imagine!’ Her eyes sparkle.

This is fantastic, of course. A tiny part of me wishes that she could have received this call about four hours ago, when we really needed it, but hey – it was only a Vogue cover shoot. What’s to stress about?

Amanda Elat calls from Miss Teen next morning. Vogue liked the editorial shots of Jenny for the six-page spread inside, but they’ve decided to use Kate Moss on the cover. I’m not surprised.

Crow finally admitted it, after we’d dropped Jenny off at her flat.

‘I love her and everything. I mean, she’s really special and great. But next time, shall we use a professional?’

And, despite the fact that Jenny is my best friend from primary school, and my favourite person to look at in Crow’s clothes, and using her was my idea, I said, ‘Absolutely.’