Maybe I could set myself up as the 5th emergency service or something—Savvy Dunn, all you need for your every online emergency. Maybe that's what I could do when I leave school because, if you think about it, everyone is online these days and most people need help to manage all their social media accounts.

I started thinking about it now because I have been summoned by Cheryl for an Instagram emergency and luckily I am within easy reach. I get the bus to Cheryl's house and dream up names for myself. Insta Fixer? All Your Instagram Solutions, Sorted. Needing Celebrity Contact? I'm your woman. Needing a thousand more followers? I can help. Wanting loads of re-Tweets? Super Savvy to the rescue!

Cheryl is waiting for me at the bus stop, jiggling from one foot to the next. This must be serious.

"What is it?" I ask, as we head back to her flat.

"Matt Rogham!" she bursts out, thrusting her phone at me. "He's sent me another direct message! I need you to reply because I'll just send him something stupid. But you must do it quickly because he sent me that message, like—oh hell—twenty minutes ago!"

I study the phone in concentration as Cheryl half walks and half-runs beside me, pulling at my sleeve to get me to hurry up. Inspiration isn't coming thick and fast (I'm distracted by thoughts of Jonathan and rumbling hunger in my belly) but by the time we are in her bedroom lying on her bed—unmade as always—I'm beginning to get a few ideas of what I/Cheryl might say to him.

"Hurry up Savvy!" Cheryl hisses, prodding me in the side with her elbow.

Matt's message to Cheryl was: "Hey gorgeous! Missed you the other night. How was ur party? I added 20kg to the bar #mannedup."

I don't think we need to hurry too much sending him a message back, as it seems to me that he's keen on Cheryl, sending her all these private messages and trying to impress her with that reference to the barbell after Cheryl (I mean me) told him he should add more weight to his work-out. I smirk to myself as I think little ol' me holding all this sway over a Hollywood film star.

I say this to Cheryl, but she shakes her head furiously. "No way! He'll think I don't like him, or I don't care if we don't reply quickly. C'mon this is URGENT!"

"Does Hot New Lad know about this?" I ask, seeing as the last time I looked, Cheryl was doing despicable Hot New Lad Danny, who stayed at the party long after everyone else had gone.

Cheryl sits up and swings her feet round so she's facing me.

"Nah, of course he doesn't, private message innit? But this doesn't count, does it? I mean, this is like, Matt Rogham—Matt Rogham, famous Matt Rogham. Danny's great, but I mean, I could end up being Matt Rogham's girlfriend! Imagine what that would be like Savvy!"

That quietens us for a few seconds—both trying to imagine what the life of a celebrity girlfriend would involve. All good things, right?

"You'd need to leave school," I say, eventually.

"Well, duh!" she replies. "Course I would. He couldn't hang round our shitty school waiting for me to come out, could he?"

I laugh. "Maybe he would and you'd be papped coming out. You'd have to start wearing big sunglasses all the time. Even indoors. And the paps would try to interview me."

I sit up too, pretending to hold a microphone in front of my face and putting on a fake voice.

"So, Savannah, what can you tell us about Cheryl?" I adopt my best serious interviewer voice. "Thanks for asking, Kelly. Cheryl's been my best friend for YEARS, but she's a real woman of mystery. I hope Matt's not itching too much, though, 'cause there is that nasty–" I break off, laughing, as she kicks me.

"Shut up you bitch!" she shrieks, and we hear her mother shout up the stairs, "Cheryl, don't you dare use that language in my house!" Cheryl flicks the vees at the door and pulls a face.

"But seriously—it would be so cool. I'd leave school. You could too, you could be my assistant or something?" She raises her eyebrows.

"Oh, thanks," I say. Until a week ago, I'd have loved that suggestion. I was ready to leave school last year, but Mum didn't agree. Or if I left school, she said, I needed to start paying my own way. Rent and food money. I ask you!

"No, you could," she insists, reaching out a false-nailed hand to clasp my arm and add gravitas to what she's saying. "I'd still need you to do my Twitter and Instagram stuff and everything, wouldn't I, 'cause I'd have millions of followers! God, we'd go to all these amazing parties, and we'd meet tonnes of other celebrities. He knows EVERYONE. Maybe I'd get asked to be in a film or something, or maybe I'd get my own reality show—The Only Way is Cheryl, but much better because I'm so much younger than those ancient, orange slags."

"And better looking," I add, loyally.

She smiles and leans over, giving me a kiss on the cheek. "Aw, Savvy!"

"And you could do your own YouTube channel," I add in, seeing as I am the social media/IT expert here. "What about Cheryl's make-up tips? Then we'd get sent hundreds of free stuff to review."

Cheryl nods along enthusiastically, liking this picture of the girlfriend of Matt Rogham life I paint. Why wouldn't she? The glitter and glamour of it is one hundred percent different from our real lives. Then, she pulls herself together. "But we've still not sent a DM back to Matt. Savvy—get a move on!"

I sigh and study Matt's message again. I follow quite a few people on Twitter and Instagram—quite a few celebrities, but my favourites are the ones who always post up smart remarks, smart arse you might say. Matt Rogham isn't one of them. He usually posts stuff up like, "Mornin' all", or "Just dun a KILLER work-out" or something about whatever film or TV programme of his is out at the moment.

The latest thing he's put on Instagram is a pic of him with another big name celebrity on the set of his latest film. They're arm wrestling and Matt appears to be winning.

"The party was... immense. Bit like your bulging biceps, I guess?" I type, and Cheryl bursts out laughing beside me.

A DM comes back almost straightaway. "Gotta get ripped for the next movie. I'm gonna be topless in it a lot."

I mull over the reply to him. I'm guessing to myself that what he wants me (I mean Cheryl) to say is, "You don't need to get ripped, you look amazing already", or "Can't wait to see you topless", and other suck-ups. But the point about the Cheryl I have made up is that she doesn't do that kind of bullshit. She's a treat 'em mean, keep 'em keen girl, and he seems to lap it up.

"Yeah, you've got A LOT work to do, soldier." I reply. "Drop and give me twenty."

Cheryl wails "NOOOOO!" as I press enter, but I shush her. "Trust me, Cheryl," I say, "This will work."

We both stare at the phone. It doesn't move, the static screen unnerving as the minutes stretch out. I start to get nervous. Maybe I've made a bad judgement call and I should have sucked up to him about his bulging biceps and topless torso after all.

Then, a film appears. A topless man in a gym pushing out press-ups quick as you like as a beefy guy with a shaved head and tattoos barks out the numbers, one, two, three. Another DM. "Just done it. Made it fifty, and I imagined you underneath me."

OMG. Me and Cheryl stare at each other. She grins, "Now, what?"

The words flood into my brain—a smartarse reply, a clever pun and some witty comebacks. Sometimes, this happens to me. I start slowly and then suddenly all these words and ideas come into my mind. I type him back something quickly and send it. I reckon it took me less than 10 seconds.

"Soldier, you felt rock hard... Your chest I mean."

His reply takes a little longer, but soon we're going back and forth, back and forth and the words get more and more outrageous. I hint at stuff and he takes it and hints back; hints back strongly.

Cheryl is watching me and sneaking looks at her account on my phone at the same time. "Hey, look Savvy!" she says, thrusting the phone in my face. Matt's tweeted something to his 3.2 million followers.

@cheryl2002 fab lady y'all and well worth a follow!

OMG. Again. We watch as Cheryl's followers multiply. 3,013, 3,014, 3,015, 3,016—a minute later and she's reached 4,000 followers. Her Twitter account has gone mad too, and her followers now include quite a few other celebrities. Cheryl has a huge grin on her face, but I'm worried.

"God, we'll need to post up cool stuff all the time. You've, like, got an audience now. You're practically famous."

"Yeah, awesome," she says. "You can do it for me though can't you? You know cool things and you always sound clever, so just post whatever up. And you can just do it whenever."

"Like when you're not there?" I ask, for clarity.

"Yeah, that's fine."

In my head, I start planning how this might work. I could write out Tweets and get pics for Instagram in advance and schedule them, and link up the two accounts, or I could put together a plan of what I'm going to put online and when. That way, it wouldn't be too much extra work. I mean, there's also my exams and shit...

Yeah, my exams. I push the thought out of my head as it's makes me feel sick. Or not sick, because immediately I also thought about food and then I imagined a huge chocolate cake slathered in fudge icing, heated slightly and with some squishy cream melting down the sides.

I'm glad I'm at Cheryl's. If I was at home... The Great British Baker decided to practice her Choux pastry skills the other night. In the fridge are ten eclairs, topped and tailed with caramel sauce and filled with creme anglaise. So far, I haven't touched them, even though they scream at me every time I walk in the bloody kitchen.

Cheryl rolls off the bed. "Are we good to go?" she says. "I'm supposed to be meeting up with Danny."

I raise my eyebrows and she mimes, "What?" back at me, a fake innocent look on her face. This could be confirmation she and Danny are a couple. Great. My best friend and my least favourite guy in the world. Loyalty, Chezza I mutter to myself.

"What were you going to tell me, anyway?" she asks. "You said you had loads to tell me when I called you yesterday."

So I did. Time to tell her about Sandy and the 'date, or Jonathan and what happened in the park, or what I feel right now, the hunger that sends sharp jolts through my belly and the way my mouth's dried up completely; the wish I could share this with someone. Is it normal to do this around food, to the girl who eats half a burger and THEN THROWS THE REST AWAY.

But after Matt Rogham, it all seems lame—apart from the Jonathan stuff but that is definitely off limits. I love my friend, but she has the biggest mouth this side of northern hemisphere.

Even so, I toy with telling her about Sandy. "Hey Cheryl, guess what? That new guy Sandy, the one you haven't noticed? He asked me out a few days ago. I know he's not Hot New Lad-ish enough for you, and I think he's changed his mind by now because I've still not got back to him, but still... Someone liked me enough to ask me out! Fab, huh?"

Something tells me Cheryl would be rude about him, or she would say something the next time she saw him that would make it obvious to him I'd told her all about it. He'd be embarrassed and maybe a bit hurt. What do I care about that though? I remember him helping me clean that office and I decide to keep my mouth shut.

"Nothing important," I say. Secrets are good, remember?

As I wait for the bus to go back to mine, I count this week's achievements. MyFitnessPal has flashed that warning at me three times already this week. You ain't eating enough, it booms, and the flash makes me want to jump up and down. When I suck my cheeks in, the face in the mirror winks back. Humpback whale? Stuff you. And today, I've spent an hour in direct conversation with a major celebrity. Even though he thought I was someone else. When he typed those messages back and forth, in his head he saw Cheryl—slim, pretty and now witty, thanks to me.

Is he in love with her? It feels that way. Every time I log into Cheryl's account, he's there or not far away and Cheryl's phone pings so much even she's fed up of it and switches it to silent most of the time.

I charge up the stairs to my bedroom, desperate to log on.

Savvyslim: Hey Annies, what has your best achievement been this week? Let's congratulate each other on what we've done.

FatGirlThin: Got my thigh gap back. Gonna post pics on Instagram soon. Follow me - @thinnyminnie

Mali-minx: Three days, three meals – only 200 cals each! Beat that!

FatGirlThin: Mali-minx, u need to try harder. ;) 100 cals target 4 next week, dead easy. Cant get thigh gap with 100cals.

Mali-minx: FatGirlThin, I got thigh gap, bitch. Ribs stick out too AND periods stopped for a year. Beat That!!!!!