As children's authors mingled with the guests, the biggest reception was given to JK Rowling, mobbed by children desperate for an autograph from the author of the Harry Potter wizard sagas.

"I think it is a wonderful idea to celebrate the Queen's 80th birthday by celebrating children's literature. I really do think it is a golden age at the moment," she told reporters at the most surreal garden party ever staged at the palace.

"I am feeling sad as it is the last one. But so far, so good."

Pressed on when the book would be ready for publication, she would only say: "I'm doing well I think. You can never really tell till you get near the end. I am not quite there yet."

Unabashed by the frenzy of young fans stretching out eagerly for a signature, she said: "This is the bit I like. I like meeting them."

Hailing what she called a golden age for children's literature, she said: "I think this is a fantastic way to celebrate the Queen's birthday. I think it was such a good idea. It's wonderful to see all these children here."

"I must say it is quite surreal to be here in Buckingham Palace," she said, disappearing behind a sea of little hands waving autograph books and Harry Potter paperbacks.

“We are in a golden age for children’s literature in Britain. I always believed children would come back to enjoying books — and that’s not just down to me.” Harry Potter would have been “a bit fazed” by the event, she said. “But he’s quite cool, and I think he would have been impressed.”

[The scene opens with Hedwig flying to Hogwarts, dropping a letter on one of the chairs in the Gryffindor Common Room. Harry, Ron and Hermione enter the room in mid-conversation.]

Hermione: But that doesn't make any sense. Hags aren't vegetarian...

Ron: Hermione, it's a joke!

Harry: What's this owl doing here? Why isn't it in the owlery?

Hermione: Must have flown in through the wrong window. There's a letter.

[Harry picks up the letter and opens it. They all sit at a table.]

Harry: I can't make it out. It's all smudged. I think it's from the Minister.

Ron: What's he ever done for us? [Takes his wand from his pocket] I'll get rid of it. I've been practising my conflaguration spell.

Hermione: Wait a minute. [Hermione takes the letter away from Ron] It doesn't say Minister for Magic. It says Prime Minister.

Harry: What, the muggle one? What's he want?

Hermione: [Reading the letter] To whomever it may concern, as long as they can use magic...

Ron: Sounds a bit desparate...

Harry: He's not being very choosy, is he?

Hermione: [Continues reading] A terrible crime has befallen the country. Her Majesty the Queen's handbag has been stolen.

Ron: Lord Voldemort's getting really inventive.

Harry: I dunno, I can't really see Voldemort with a handbag myself.

Hermione: There's more. [Continues reading] This handbag contains the reading glasses her Majesty is due to use to read her speech at her 80th birthday celebrations.

Harry: That's today isn't it? Peeves was using it as an excuse to drop red, white and blue eggs over everybody at breakfast. [Takes the letter off Hermione and continues reading] We urgently need you help. We don't know who could've taken the bag or how to get it back. We've tried everything. [Finishes reading] I know they use veritaserum to make thieves at the Ministry confess.

Hermione: But we don't know who the thief is.

Harry: Oh yeah, good point.

Hermione: There must be another way. It can't be that difficult.

[Harry gets up and points his wand at the table]

Harry: Accio lost handbag!

[The spell produces nothing. Harry indicates for Ron to come and help.]

Harry & Ron: Accio lost handbag!

[A heap of handbags appear on the table]

Harry: It's bound to be one of these.

[They start going through the bags. Hermione finds a stuffed bird and Harry finds a sandwhich. Neville enters the room.]

Neville: What are you doing with my Gran's handbag?

Ron: Hey, Neville, your Gran isn't the Queen, is she?

Neville: What??

Hermione: Ron, none of these handbags are the Queen's. We're too far away from Buckingham Palace to work the Accio charm.

[Ron meanwhile is still searching through a bag]

Neville: I wouldn't put your hand in there, Ron.

[Ron gasps and lifts his fingers out. They're caught in a mouse trap.]

Harry: Who keeps a mouse trap in a handbag?!

Neville: My Gran, ever since she put her hand in her bag in 1947 and found a fanged gerbil in there. My Grandad thought she'd find it funny.

[Ron grimaces in pain, trying to get it off]

Hermione: I've got an idea.

Ron: [Thinking she's on about his hand] Brilliant, could you hurry?

[Hermione goes to another table and sits down, writing a note]

Ron: I'm losing the circulation in my finger.

Hermione: We'll send a message to the Palace. It'll just mean a temporary transferance of magical power. We can easily do that.

[Harry gives the note to Hedwig]

Harry: Off you go! Buckingham Palace! You can't miss it. Big white place with a flag on top.

As he soared over the palace walls he set off an alarm and armed cops swooped within seconds to protect Her Majesty.

Ook flew into nearby Hyde Park, where the Wireless O2 music festival was being held.

The din meant Ook — who plays Harry’s pet Hedwig in the Potter films — couldn’t hear his trainer’s whistle.

A source said: “He started a real commotion. The police arrived in seconds after he flew off. He made himself scarce and his handlers and trainers went to Hyde Park to look for him.”

The source said: “We had a spare, but we’re delighted to find Ook. He’s precious.”

“Well, what’s interesting about the Potter phenomenon is that my grandfather’s books came before,” says Dahl. Are you saying she ripped him off, I ask, thrilled at the prospect of a stand-up fight between the two women at Sunday’s garden party?

“Well no,” she says carefully, “but JK Rowling has been very generous in acknowledging how she was influenced by him, and you can see it in Potter. My grandfather was probably funnier than her, and she is probably more epic, but it’s clear how like him she is in other ways. She’s interested in good and evil and, like him, isn’t at all afraid of the dark side of life.”