Some of Princess Diana's most iconic Catherine Walker outfits are on display in a new exhibition

Catherine Walker & Co is celebrating the brand’s 40th anniversary with a three-day private exhibition featuring several dresses worn by their most famous client, Princess Diana. The event, which spans four decades of fashion design, further reveals the close relationship between the late royal and her favorite fashion house.

One of the outfits, a stunning white lace halter-style evening dress with embroidered pearls, sequins and crystals worn by the princess to a gala in Washington in 1996, is displayed for the first time. Alongside it sits a heartfelt note of praise from the then recently-divorced Diana.

“I was so proud and felt very confident to stride out there and deliver my first speech since the divorce,” the letter reads. “The compliments about your design and expertise would have made your ears burn.”

Credit: Simon Perry

On an exclusive tour of the exhibit on Monday, Said Cyrus, the brand’s co-founder, said the gown “gave her great confidence when she stepped out after her divorce.”

The Princess Of Wales Visits Washington
Credit: Julian Parker/UK Press via Getty Images

It is one of the stand-out designs associated with Diana that is on show — for invited guests only — at the magnificent Spencer House, Diana’s ancestral home, in London. Also included in the exhibition are dresses worn by royal cousin Gabriella Windsor and a preview of their new AW17 collection, 20 pieces that reinterpret classics for which the house has become renowned, alongside coats, evening wear – all of which show off the house’s exquisite tailoring.

Cyrus and his wife Catherine Walker (who died in 2010) founded Catherine Walker & Co in 1977. In the 1980s and 90s, the design duo became famous for dressing Diana. They continued to work with her after her divorce, although Cyrus revealed that towards the end of her life, her outfits were borrowed rather than bought.

“When she returned them, we had no idea at the time that there was anything special about them,” he said. “But obviously now we look at them and we treasure them.”

Another outfit of Diana’s on display is a pale pink skirt suit worn during a visit to a hospice. Cyrus says the design of Diana’s clothes were often intentional and she wasn’t looking to intimidate others with her clothes.

“She didn’t want to wear something which was overpowering so the balance between looking like a princess and looking friendly was something which my wife and I looked at quite a lot,” Cyrus said of the ensemble. “The gold buttons give it a slightly playful touch without it being overpowering.”

The husband and wife team gave careful thought to the entire outfit, including how it would look from the back. “We were always very aware that wherever she was being photographed, she was seen from every angle. The back was just as important as the front,” revealed the Chelsea-based designer.

Several decades later, Diana’s daughter-in-law Kate Middleton has become a loyal customer of the fashion house: She often chooses their signature tailored coat dresses for official engagements and royal tours. And while Cyrus won’t speak specifically about his relationship with Kate, he tells PEOPLE: “We always remember that it’s not actually the dress that’s the star – it’s the girl wearing it.”

Catherine Walker’s milestone anniversary comes in the same year as the 20th anniversary of Princess Diana’s death. And two decades later, Cyrus speaks fondly of his time spent with Diana.

“She was extremely polite, very courteous, very humble,” he said. “She was never late for an appointment and she always wrote us a note to thank us when she wore something that was particularly admired.”

The private show runs every evening until Oct. 18. Ticket sales will raise money for Brendoncare, a U.K. charity for senior citizens, and SSAFA, an armed forces charity.