It's called "Growing Up Again" by Catriona McCloud.
Janie Lawson's life hasn't turned out quite the way she'd hoped.
Nearly forty, she's in a marriage that's frozen over with a mother-in-law she despises. Before Janie can take the final step towards divorce, though, her fate is taken out of her hands. Janie wakes up in her old bedroom and finds it just as it was in her teens. But it isn't until Janie looks in the mirror, to be confronted by her fifteen-year-old self sporting the most diabolical 80s perm, that the reality of her situation sinks in. She's been swept back in time to 1981 and all the signs suggest that it's a one way trip. But there's an upside - Janie has the chance to make her life turn out the way she wanted it. She's determined to help her parents, make her fortune and do some good in the world, starting with saving Lady Diana Spencer from her fate. But pretty soon Janie realises that even second time around, nothing is guaranteed...
This doesn't even begin to cover how good the book is. The fact she remembers everything is perfect, and there's no worrying about exactly what will happen when she changes things. Sure, there's a little grief at the fact that William and Harry might not get born, but other than that she's single-minded in the pursuit of her goals. I love the postcards she gets, the fact she uses her 40 year old adult self's sarcasm on her teachers and the way she spends most of the book trying to remember her hamster's name!
I actually had to set my phone alarm so I wasn't late back off my lunch cos I was so engrossed in this.
And no...I won't lend you the proof cos I'm worried I wouldn't get it back!