For many it is August, not April, which is the cruellest month. The followers of Diana Princess of Wales, who died on 31 August 1997, have not diminished in number but increased in the years since that unspeakable end in a Paris underpass. At this time of year new generations - less critical, less informed, but just as eager - rest in the shelter of her lengthy shadow, and ponder her legacy.

In London, there's still an annual pilgrimage attended by young and old to the gates of Kensington Palace, but more appropriate in these less emotionally-charged days is a trip to the Diana Memorial Fountain in Hyde Park. For so long a symbol of controversy - its design, its cost, its efficacy as a monument all angrily called into question - it has, like the woman to whom it is dedicated, lost its sharp edges with the passing of the years.

Cynics deride the deification of Britain's lost icon, but they lose the argument both on emotional and on practical grounds: the princess remains for ever young in memory, her image a rallying-post for those who subscribe to Keats' dictum on truth and beauty. You can question the poet but you cannot deny him; and so it is with Diana.

And while her life stopped short, the good works continued. The Diana Memorial Fund has handed out more than £100 million to over 350 good causes and, despite occasionally losing its footing, has much of which it can be proud.

And thus Princess Diana lives on, out of reach but not out of sight. Those who winced at the parade of her engagement ring on the announcement of Prince William and Kate Middleton's nuptials have come to recognise in the prince a silent but rugged determination that his mother's name should not be shut away. So very angular a gesture then seemed inappropriate, but as time goes on it becomes ever more clear that in William's time Diana's name will rise again - Kate Cambridge now not only wears the ring, but the ear-rings too. It doesn't take a crystal ball to guess where that scenario is going.

Sentimentalists who care to touch the hem travel to Althorp at this time of year to see a two-month exhibition which, to a degree, brings life to the Diana story. Clothes, artefacts, letters, diaries, mark out her life in a way which points to the appropriateness of a more substantial fixture, but the virtue of this display is that others around the globe get to see it too - so far it has travelled to the USA and to Hungary and is being viewed by record numbers.

So though the years pass, the power of Diana does not diminish. What sets her apart from other icons of the 20th and 21st centuries is there is an innate goodness, a wholesomeness, which attaches itself to her name: even though we recall the indiscretions, the bitchery, the bad calls and the barely-controlled emotions of the woman, she has emerged perhaps unexpectedly as a force for good.

It's best to remember her that way. Long ago, the British royal family had a much-loved icon in the shape of Prince George, Duke of Kent (1902-42). On the day of his wedding in 1934, as many people crowded the Mall as they did for Diana in 1981, and for William this year.

For reasons best known to themselves, the House of Windsor chose to airbrush their gilded son from the history-books, and he is unknown today.

It can never happen to Diana. Her name and her legacy continue to grow in an elegant, exponential, curve.