AT noon today, 41 guns will line up to salute the Queen in London's Hyde Park, and 62 more will be at the Tower of London, to mark her 75th birthday. But this will be the only official event to celebrate the milestone.

While you might be forgiven for thinking there will be some lavish ball or huge dinner party thrown to honour her birthday, the Queen will spend the day quietly with the Duke of Edinburgh at Windsor Castle.

This is a direct contrast to her 70th birthday when family and friends were invited to a private dinner party in a restaurant overlooking the Thames.

And one birthday that she will always remember is her 21st which she spent in South Africa with her parents, George VI and Queen Elizabeth, and sister Princess Margaret.

She was in love and had been reluctant to join the trip, but was delighted on her birthday when her father gave his formal consent for her to marry Prince Philip.

Although this year's birthday celebrations might appear to be muted, Harold Brooks-Baker, publishing director of Burke's Peerage, says she will see the quiet interlude as a rare treat.

He says: ''The majority of us like to celebrate our birthdays by having a party, going for a meal or doing something which is different to our normal routine. And, in this respect, the Queen is no different. She is normally so busy having to carry out her duties that spending some quiet time at home is her idea of doing something different.''

The majority of her immediate family are in other parts of the world on her birthday. The Duke of York is in South Korea on an official visit, and his sister Anne, the Princess Royal, is in central America, also on official business.

Prince William may telephone his grandmother from Africa where he is spending part of his gap-year before starting university at St Andrews in September. His father, the Prince of Wales, is currently on holiday in Scotland at Birkhall on the Balmoral estate.

Neither Edward and Sophie, the Earl and Countess of Wessex, the 100-year-old Queen Mother or Princess Margaret are expected to be at Windsor Castle this weekend.

Peter Archer, PA News' Court Correspondent, says in recent years the Queen has been cutting down on the number of official duties although this does not necessarily mean she is taking things easy.

''She only does two state visits each year, one is in early spring and the other is in autumn. She has become more selective and has handed some of the royal duties over to the Prince of Wales, Princess Royal and the Duke of York.''

Brooks-Baker adds: ''Although she has cut down on the number of duties she carries out she still does have a busy work schedule and does not have much time to herself. She will be on the throne and continue to carry out her duties until she dies. I do not think there is any question of her stepping down.''

The Queen has a healthy constitution and her only adult health problem is sinus trouble, which she treats with homeopathic pills containing grains of deadly nightshade.

Her leisure time activities reflect her love of sport and enjoying the great outdoors. ''She still loves to go riding when she can,'' says Brooks-Baker. '

But how do our experts think the Queen will look back on her reign up to the present day?

Archer says. ''Any mother would be saddened by the fact that three of her four children's marriages have ended in divorce and then there was the trauma of Diana's death.

Brooks-Baker adds: ''At the end of the day she is the most complete person to have as our head of state. What has surprised me is that, in recent polls, teenagers have said they are in favour of the Queen still being the head of state. I have even spoken to extreme left-wing politicians who declare they cannot fault her for the way in which she has carried out her duties during her reign.''