Kate Middleton forms Royal alliance with Princess Diana's favourite designer Catherine Walker

Kate Middleton's sartorial decisions have become as crucial to her success as the outings and engagements that she attends. Each choice is carefully deconstructed and then purchased by those wanting to mimic her relatable but graceful, elegant style. From Roksanda and Alice Temperley to high street selections from Zara, the labels immediately reflect the Duchess in a negative or positive light depending on the name and indeed the price.

It comes as no surprise then that Kate has returned time and again to the subtle sophistication of Diana's favourite couture design house, Catherine Walker.

After the festivities of the Trooping of the Colour, Middleton brightened up the Order of the Garter service at Windsor Castle yesterday in a vibrant red coat dress by the British designer, cementing her alliance with the brand that so carefully crafted and protected Princess Diana's image throughout the 1980s and 1990'.

To celebrate Chelsea Flower Show last month, Kate chose a neat, modest but vibrant coloured long sleeved dress by Walker to attend the horticultural celebration. This was not the first time she had worn this dress, previously stepping out in the design in 2014 during a royal tour of New Zealand and Australia. For the Queen's 90th birthday thanksgiving Church service, the 34-year-old opted for an ice-blue embellished dress coat by Walker.

In a departure to her more directional choices, Walker's designs are by no means trendsetting and cannot be bought by the regular shopper.

Following a formula of long sleeves, neat circle skirts and neat and subtle adornment that are perfectly fit for purpose, Walker's designs protect her identity as a member of the Royal family rather than a fashion status symbol. Similarly, Princess Diana relied on Walker throughout her years in the public eye and was buried in a black dress by the designer.

French born Catherine Walker, who died in 2010, opened her couturier in London's Sydney Street back in 1977 with the aim to return to traditional Parisian couture style – where a fashion house relied on a close relationship with the client.

Working with her husband Said Cyrus, who runs the business today, Walker's focus was on careful, personal design and discretion. In an interview with Kensington and Chelsea magazine in 2012, Cyrus cited their motto: "what was said in the workshop, stays in the workshop". Ultimately it was "the client who became the star of the show, not the dress". It comes as no surprise that Diana forged an alliance with the designer who would come to create over 1,000 dresses for her.

The infamous sequin gown that Walker designed for Diana recently went up for auction with the expectation to sell for over £100,000. The dresses that Walker created for Diana are certainly not evergreen in style but they are undeniably iconic. The role of these designs was to become part of a legacy – demonstrable in the upcoming auction price of this sequin creation.

The princess won over hearts and cameras in the showstopping glittering creation several times including the Vienna Burgh Theatre gala in Austria in 1986 and the Diamond Ball in 1990. Again, while many would be afraid to recycle such a memorable dress, the design became part of her image rather than just a flash from an event, something that Middleton has mimicked and mastered.

Diana had no shortage of access to luxury designers, and Gianni Versace, Ralph Lauren and Christian Dior all found their way into the Princess's reportoire in the same way that Middleton stuns in the likes of Roland Mouret and Alexander McQueen.

Contemporary choices may elevate their style but Catherine Walker's absence from the public eye allows the wearer to be the star of the dress rather than the other way round. While Middleton's vibrant Walker creations may not be catwalk fodder, they are synonymous with her and no one else.

The Diana effect may have secured Catherine Walker's heritage but the Kate effect has ensured the brand's continuing relationship with new royalty – seen and admired sartorially, and without judgement.