In July this column intended to give up boy wizards as well as smoking in public places – but then I found myself speaking at the tenth-anniversary Harry Potter convention in London: Sectus 2007, held on the weekend when our world was convulsed by book seven. It was a weird experience to babble on-stage about my spinoff volume in full knowledge that nine hours later, at one minute after midnight, this book would be magically transformed into a pumpkin.

Sectus had a peculiar innocence, like early Discworld conventions before the fans realized they could skip occasional programme items to socialise in the bar. On the Thursday evening I sat bemusedly on a bench outside a deserted pub near King's Cross, watching a very, very long queue move very, very slowly into the Camden Centre for the Sectus opening ceremony.

They nearly all seemed to be women: apparently less than 20% of the 450 members were male. Most were interestingly dressed. With me on the bench was the noted Irish SF fan James Bacon. "God," he kept saying Irishly, "Lookit THAT girl with the knee socks. Aren't those school uniforms GREAT?" He dribbled particularly about women with loosely knotted school ties dangling between their bosoms. It was like a flashback to the sixth form at St Trinians. Did I mention that no under-18s were permitted?

As a guest speaker I was allowed to jump the queue and lurk in the bar/buffet while fascinatingly clad ladies (and a few chaps) lined up for authentic, institutional "Hogwarts" school meals. In the auditorium, long before anything was actually scheduled, huge crowds patiently watched an endless slide show of Ministry of Magic directives. My sole duty that night was to get introduced at the opening ceremony, which seemed just about within my capabilities ...

Next day any sensible person would have stayed home during the tropical storm and floods that hit England, but I am not a sensible person. I staggered dripping into the University of Westminster, clutching a much-rewritten speech titled "Hogwarts Proctology Class: Probing the End of Harry Potter" and thinking profound thoughts like "Oh God!" James Bacon was on the prowl, eager to photograph dishy young ladies. His cunning ploy was to ask the leggiest ones if they'd mind posing with this famous guest speaker. Your columnist was too damply weak-willed to resist.

That university venue seemed strangely unpopulated, because virtually everyone was earnestly listening to talks like "The World Turned Upside Down: Harry Potter and the Queerness of Children's Literature." I chatted to a long-time Star Trek fan who was reminded of the early, mostly-female 1970s Trek conventions. Her t-shirt was blazoned with a short-skirted woman (or so I thought) with long black hair and a nasty expression. She had just bought the original artwork for this. Er, who was it? "That's Severus Snape in a little black dress," she explained patiently, and again I realised there are mysteries of the Potterverse with which I shouldn't meddle. Indeed the Sectus art show had enough material inspired by slash fanfiction to make me reconsider thoughts about innocence.

My own performance ("testing, testing, Imperio, Crucio, Avada Kedavra ...") seemed to go well enough and they laughed at some of the jokes, even the unreliable predictions that Harry would find true love with Dolores Umbridge and learn his father was in fact Darth Vader. Splendid people. Still, it was a relief to finish without developing symptoms of the Bat-Bogey Hex.

Time was running out as the ten-year phenomenon built towards its final splurge. As I paddled home through a maze of rail cancellations, Sectus awaited its midnight delivery of 300 copies of book seven to be swotted up as homework for the Saturday-morning classes in Applied Pottery which – as early as 10am – assumed the whole audience would have read it. The spoiler and non-spoiler convention zones were carefully marked; a system of coloured ribbons and "I Have NOT Read The Book" stickers identified fans unwilling to hear who got killed off. Several suspiciously red-eyed folk had already squinted at the thing as blurry digital images on line. You don't see this kind of frenzy when Martin Amis publishes another novel. Or even Dan Brown.

Meanwhile the entire book trade was moaning that, thanks to heavy discounting and supermarket pressures, this would be a colossal worldwide bestseller from which none of them made any actual money. Funny old wizarding world, eh?

David Langford was relieved not to be too embarrassed by plot developments in The Welsh Holly Data (anagram).