The Duchess of Cambridge isn"t usually one for following trends, but by steppingout in a new Erdem grey coat,matching John Boyd hat and Rupert Sanderson heels she"s the latest person to buyinto fashion"s current vogue for Groutfits - a head-to-toe grey look that"s proved so popular it now has it"s own tumblr, FiftyShadesOfGroutfits.

But if her Erdem lace-detail coat for the Commonwealth Service at Westminster Abbey was on trend,she accessoriedher look with her most traditional hat yet; a wide-brimmed creation by Princess Diana"s favourite milliner John Boyd.

"John has been making this shape since the 1940"s - it"s a classic, flattering brim," the people at Boyd"s Knightsbridge shop toldThe Telegraph of the Duchess"s hat.The 90-year-old opened his shop in 1946 and remains the oldest working milliner in London.

This is the first time that Kate, whonormally opts for fascinators, has worn one of Boyd"s creations.Thecustom wide-brimmed grey peach bloom felt hat with a silk dark grey trim would have taken up to a week to make, with everything hand-stitched by Boyd"s three-person team.Similar styles retail for ??500.

Duchess of Cambridge: every single outfit

But whilst the Duchess is new to working with Boyd, the milliner has a long-standing relationship with the royal family.Princess Anne first wore his designs when she was 16 and then commissioned him to makea yellow and white net saucer shaped hat forPrince Charles and Princess Diana"s wedding.And Boydwas credited with sparkinga "hat renaissance"when he designed pieces for Diana, the Princess of Wales, in the 1980s.

He first worked with Diana onthepink tricorn that she wore with her "going away outfit" after her wedding in 1981."My new range was just being completed when Diana arrived, and sitting on a chair surrounded by ribbons suddenly took a tricorn shape in sparterie that was hanging on the door and said, "I like that style, I will have it for my wedding,""Boyd has said of working with the late Princess.

He went on create some of Diana"s most iconic hats and witnessed first-hand the impact they had on the public.???Mothers would drag in their daughters with long faces shouting ???God, you???ll look a scream in that!???, then suddenly they all loved it???hats with feathers, ribbons and veils like the Princess of Wales,"the milliner has said of the Diana effect."Diana was our best ambassador for hats and the entire millinery industry owes her a debt.???

So will Kate"s wide-brimmed hat have a similar impact?"There has been a return to classic wide-brim styles recently, especially at this time of year when racing and wedding seasons approach," Boyd"s representatives toldThe Telegraph- before offering some crucial advice for anyone thinking about copying the look."We always think it"s nice to tip the brim down to the right so it frames the face.Most hats tip over the right eye, it"s a traditional thing."

Princess Diana Kate MiddletonEmma ???7 mins