She's seen the show 41 times. So what is the secret behind the phantom's mask?

For a reputedly shy chap, Sir Andrew Lloyd Webber does not do things by half. Tonight, to mark the 10th anniversary of The Phantom of the Opera, he will be giving a masked ball at a London hotel and even taking to the stage at Her Majesty's Theatre to do a surprise turn after the curtain calls.

Sir Andrew and the musical's producer, Sir Cameron Mackintosh, have sent a stylised invitation to 10th-anniversary guests purporting to be from the Phantom himself. "Masks must be worn. Ignore this at your peril" it concludes.

In the case of Messrs Lloyd Webber and Mackintosh, that is, one assumes, a tongue-in- cheek affectation. For others though the magical powers of the Phantom and his infatuation with his protege Christine Daae in the 19th-century Paris Opera House have been tangible enough to take over their lives.

Take for example Miss Christine Daae herself. Not the fictional diva who inflames the Phantom to murder for her love in the swirling Gothic romance, but the 22-year-old PA from Bishop's Stortford in Hertfordshire who changed her name from Victoria Bohm by deed poll.

Miss Daae of Bishop's Stortford has seen the show 41 times, once travelling to Canada to do so, and has spent around pounds 6,000 on tickets and merchandise. The show, she said yesterday, "totally took my breath away. I felt completely carried away to another world, caught up in the hypnotic power he has over Christine".

She explains the change of name in appropriately romantic style. "Unconsciously it was that if the Phantom came back today he would have a Christine Daae who would stay by him at the end," - a sideswipe at the original Christine who deserts her mentor once nightly and twice on on matinee days.

Miss Daae's identification with the piece is not unique. Ushers at Her Majesty's Theatre regularly see people sitting in the stalls in phantomesque masks.

And on one occasion audience involvement nearly led to an international incident. Ushers were shocked to see the Queen of Sweden enter the theatre's VIP room at the interval, hair in a mess, clothes dishevelled. Her bodyguards had mistaken the coup de theatre at the end of the first act, when a chandelier crashes from the ceiling above the auditorium on to the stage, as an assassination attempt - and leapt on the Queen to protect her.

There are currently 10 productions of Phantom of the Opera playing worldwide. And last year in Basel, Switzerland, a pounds 25m theatre was designed and built to show The Phantom of the Opera and only The Phantom of the Opera - in perpetuity.

Some members of the audience travel to the theatre on a dedicated aircraft which carries ticket-holders from all over Europe. The Corsair Saab 2000 has had its fuselage painted with the Phantom's mask logo, dwarfing the Swiss flag. As it takes off, the climax to the title track plays through the cabin.

If he studies the Phantom figures before setting out tonight, Sir Andrew will note that worldwide it has grossed pounds 1.4bn. In London alone it has taken pounds 80m and has been seen by nearly 5 million people.

The moral of which is: Always linger around second-hand book stalls. It was at one such stall in New York that Sir Andrew bought the book on which the show was based, for 50 cents. His first thought was that the story was "a load of hokum". He now speaks of it with a little more passion.

"There was a lot of love in it for me," he says. "I wrote the role of Christine for my then wife Sarah Brightman, and a lot of passion went into it."

The facts behind the musical magic

n The chandelier, made of 6,000 beads and weighing one-and-a-half tons, makes a four-second fall to the stage towards the end of every performance.

n There is almost a mile-and-a-half of drapery around the stage.

n Each dancer wears out one pair of dancing shoes every three days.

n Cast members have an average of seven costume changes per performance.

n The Phantom's mask, with integrated radio microphone, takes two hours to put on and one hour to remove.

n A ticket to watch one of the ten "Phantom" productions showing around the world is bought on average every nine seconds.