Twenty years ago, Kensington High Street was a sight. The processions from the Tube station to Kensington Palace began in earnest on the Monday morning. People with grief-stricken yet curiously self-important expressions walked ceremoniously bearing “floral tributes” and hand-written testimonies to place before the building’s forbidding railings.

By lunchtime next day the flowers were waist-high and spreading 30 feet from the gates. So it continued all week until the bouquets (“wasteful, weak, propitiatory flowers”, Philip Larkin) reached back a hundred yards, piled five feet high at some points, starting to compost down underneath.

As an installation, it was quite a thing: an outward manifestation of a permanent change in our national character, for better or worse, perhaps. This week the Duke of Cambridge remembered seeing weeping members of the public back then and thinking: “You didn’t even know her. Why, and how, are you so upset?”

Working at the Standard, in Kensington High Street, that week, making one’s way to and from the Tube through the mourners, was a strange experience. At first it all just seemed baffling: by the end of the week, moving among the grievers felt positively oppressive, even coercive. Years later I was suddenly reminded of it by the scenes in The Walking Dead, in which survivors are forced to drape themselves with rotting entrails, and impressionistically stagger along themselves in order to pass unnoticed through the ravening zombies.

There were other sites of remembrance and pilgrimage in London, and around the country, but it was clear that there was something about the whole set-up at Kensington Palace — those immense railings excluding people from the palace, appearing to imprison Diana’s spirit within — that exacerbated that feeling of outraged identification.

Rain clouds over the gates of Kensington Palace, where flowers and picture tributes to the late Diana hang (AP)

And now it’s all changed. Kensington Palace these days presents an entirely different face to the world, effectively turned 90 degrees on its axis, to face not down to the High Street but out to the Round Pond and the full sylvan expanse of Kensington Gardens and Hyde Park.

(Jonathan Brady/PA )

The architect of this entire transformation around Kensington Palace, opening it up so beautifully, is Todd Longstaffe-Gowan, gardens adviser to the Historic Royal Palaces for 25 years. “The aim was very simply to re-marry the immediate surroundings of the palace with the park itself — to open up the gardens as they were in the 18th century,” he told me this week.

The whole approach to the palace is now open and light, spacious and welcoming, hospitable and even, to look at the pleasure on visitors’ faces, just fun. While seven or eight million people walk up and down the Broad Walk in Kensington Gardens each year, only 100,000 or so used to visit the Palace Gardens. Now it’s a million. “It’s been a dramatic change,” says Longstaffe-Gowan, pointing out that now people are also allowed on to the grass.

Princess Diana in 1997 (Tim Rooke/REX/Shutterstock)

Longstaffe-Gowan was appointed in 2008 after an open competition. After several years of initially alarming destruction and construction — sweeping away 60 massive trees, countless shrubs, and much furnishing clutter to open up views; creating new gravelled walks, terraces, lozenge-shaped beds, and a wildflower meadow; surrounding the statue of Queen Victoria with a reflective pool; installing an amazing hornbeam “Wiggly Walk” up to the renovated Sunken Garden and its Cradle Walk — the gardens re-opened in March 2012. And, as they bed in, they are better year by year.

The plan had been long nurtured, “but until the death of Princess Margaret [in 2002] we didn’t have the estate in hand”, as Longstaffe-Gowan tactfully puts it.