Harry Potter probably has a spell for it, learnt at Hogwarts School of Witchcraft and Wizardry, and it would change my life for good. The spell I need is “Tempus reversum” or “Clockus backus”.

What I want is to turn back time exactly 10 years, to the day when I sat in an Edinburgh cafe opposite a palefaced, redheaded single mother who, after years of struggling, had just published a children’s book.

At the time I was presenting BBC Radio Scotland’s book programme, Cover Stories. Every day, books flooded into our office. Most were going nowhere, though once I was lured to Leith to interview a polite, softly spoken man who had written a book about trainspotting. Everything else is a bit of a blur.

But I do remember the day I was asked to go to interview that children’s author in an Edinburgh cafe. We arranged to meet there, her daughter asleep in a pushchair beside her. We spoke about the logistics of writing a book in a cafe, and about her wizard hero. She was anxious that nobody would want to buy her book.

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At the end she picked up my copy of her book and reached for a pen. “Would you like me to sign it?” Something happened then. Maybe the waiter came to take away the empty coffee cups, maybe I realised that I had to rush. Maybe her daughter started to cry. (Writing this, I may too.) I picked up the book and left without her signature. Back at the office my producer asked me how it went. Had I got her to sign the book?

“No point,” I said, confidently. “When you’ve read as many books as I have, you know instinctively when something is going to be a hit. You mark my words. We won’t hear of Harry Potter and the Philosopher’s Stone ever again.”

Not the best critical judgment of my life – in fact, the literary equivalent of the record company boss who told a mop-topped foursome called the Beatles that guitar bands were over. I am the man who thought there was no future in the talkies, who believed computers would never take off.

Over the years I have had ample opportunity to regret my moment of madness. With every Harry Potter story, lunch box, comic, cartoon and snack-time chew sold that regret has deepened. With a new film out now, and the final book out this week, I’m going through a rough time.

Undaunted by my lack of confidence in her selling power, Joanne (as I have never again been allowed to call her) has amassed global sales of 325m copies. With the movies, merchandising and the rest, she is worth £545m, according to The Sunday Times Rich List. Along the way, people associated with the Potter magic have got wealthy too: her agent, her publisher, the actors in her films. And Toby Rundle.

Rundle (there is no reason his name should mean as much to you as it does to me) is the student who sold his first edition of The Philosopher’s Stone for £7,200 at auction last month. “It should pay my way through university,” he cheerily told reporters. I hope you enjoy every minute of your time in college, Rundle.

It gets worse. In May a copy of the same book, inscribed “To David”, sold in London for an eyewatering £27,370. That David could have been me.

“People have been trading in this particular first edition for years, treating it like a commodity,” says Philip W Errington, a children’s book specialist at Sotheby’s in London, rubbing salt in my wounds.

“In mint condition, you could probably bank on getting between £10,000 and £15,000 at auction for one.” And the rest. Ten years ago, £27,370 would have been a handy amount of money. It wouldn’t go amiss today.

In the meantime life hasn’t changed much for me or for the woman I met in the cafe. J K Rowling still lives a few streets away from me in Merchiston, when she isn’t in her Perthshire castle, her London mansion or her US holiday retreat in the Hamptons.

In a spectacular feat of locking the stable door after the horse has bolted, I haven’t thrown a book out since. But none of my other books are worth much more than the price on their dust jackets, and many are worth much, much less.

If the Potter books have a message, it is about knowing your destiny and seizing your chance. At the first hurdle I failed the Potter test. I missed my destiny, fluffed my chance, didn’t see that the holy grail of first editions was right in front of my nose.

It’s small consolation to think, as Philip W Errington points out, I may have earned a small footnote in the history of one of the most famous books in the history of publishing: “Everyone would love to get their hands on a Harry Potter first edition. You are unique – you’re the only person in the world who turned one down.”

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