Joanna Hunkin: Impostor Potter

In my mind, Harry Potter's face has always been rounder than Daniel Radcliffe's. Ron Weasley is more brooding and less bumbling than Rupert Grint and Hermione's hair is a giant shock of frizz - rather than the increasingly sleek locks of Emma Watson.

Hogwarts is a rich, emporium of curiosities - its rooms draped in plush velvets and the grounds marked with manicured gardens and ornate stone buildings - rather than the imposing but austere castle of the film franchise.

Anyone who has read the Potter books will have their own version of the young wizard's world. Mine, for anyone who cares, is more akin to the original Charlie and the Chocolate Factory aesthetic. Overflowing with bright colours and mad, over-sized objects.

No matter how many Potter films I watch, or how many times (far more than anyone should ... it's my secret shame) the Hogwarts in my head doesn't change. Daniel Radcliffe will never be my Harry. That's no criticism of the films - all of which are entertaining in their own right. Rather, it's testament to J. K. Rowling's writing.

The delight in reading Harry Potter is in the detail - the colourful and convoluted descriptions of everything - from pots and pans to fantasy lands. The plot, though often exciting and engaging, is secondary. And no matter how many directors take over the Potter reins, their vision will never replace that embedded in my own mind.

In fact, I find watching the Potter films similar to watching any other sequel without a literary basis. The characters are familiar but I'm never sure what the story will bring. So many chapters are culled in the making of the film versions, they often bear little resemblance to the original books. Some fans cite this as a major criticism of the films, but I rather enjoy it. Where's the fun in watching something where you know exactly what's about to unfold?

Ron Howard did that with The Da Vinci Code, sticking rigidly to the book's elaborate plotline, and came out with the biggest snoozefest this side of The English Patient.

Though, to be fair, I'm fairly hazy when it comes to recalling each of the individual Potter plots.

After seven volumes, hundreds of pages thick, I'm damned if I can remember who died in which one or work out whether Snape really is friend or foe.

Combine the seven books with the five films to date - six as of next week when I plan to see the latest release Harry Potter and the Half-Blood Prince - and you have a whopping 13 stories to keep up with. My brain just doesn't have room for that sort of trivia.

And that's before they've turned the seventh and final book into two films, bringing the combined total to 15. Never mind the video games, books about the books, and other obscure paraphernalia.

It all seems a bit like overkill for something that started as a simple children's book. But when you consider how many books have been sold - 400 million and counting - eight films are but a drop in the ocean.

As David Yates' Half-Blood Prince is released next week, it represents just one man's vision. One of 400 million different Potter worlds locked within people's minds.