If in the world of red carpet fashion you're you're only as good as your last outfit, the Duchess of Cambridge had a lot to live up to in her first public appearance since her wedding day and that dress.

Equally anxious about her appearance before the world's press at a charity gala on Thursday night was a designer hoping for what, in sales terms, may become known as the Kate Effect.

Both parties appear to have pulled it off as Kate plumped for a glitzier number than the ivory gown designed by Sarah Burton at Alexander McQueen for her nuptials. Her full-length sequinned dress was from British society designer Jenny Packham's spring/summer 2011 collection, and – despite costing £3,500 – has already sold out.

The choice marked a departure from the more modest and classic – not to mention high-street – ensembles that she has been spotted in since she married Prince William. That is, of course, because the Duchess hasn't really been out on the tiles since she hung up her bridal gown. And the world has been waiting with bated breath to get a look at her next great wardrobe reveal (so much so that Sky News broadcast the royal couple's arrival live).

Kate's ability to empty the shelves and clear the rails of high street stores here and overseas is much publicised. The £175 Reiss dress that she wore to meet the Obamas this month sold out in stores and online within 24 hours, and public clamour crashed the company's website; supermarket chain Tesco is allegedly producing a copy of it for only £30.

The blue silk wraparound Issa dress that Kate wore when the couple announced their engagement was widely imitated, for as little as a tenth of its original price. Whistles, too, has benefited from the Kate Effect, re-issuing and renaming a blouse in her honour, after it sold-out in the aftermath of the official engagement portraits.

Ms Packham, 46 and from Southampton, is a familiar name on the British fashion circuit, yet another graduate from London's powerhouse of design talent, Central Saint Martins. The school's alumni also include John Galliano and Alexander McQueen, although Ms Packham, the sister of naturalist and Really Wild Show host Chris Packham, is known for being rather less controversial.

Her customer base includes a healthy selection of the Chelsea set that Kate is so much a part of, as well as celebrity clients such as Denise Van Outen, who was married in a Packham gown. Known for her full-length, embroidered and embellished pieces, Ms Packham's designs have also been spotted on Jennifer Aniston and Reese Witherspoon. Ironically, she is most renowned for her award-winning bridalwear, and was a strong contender with the bookies in the royal wedding dress sweepstakes.

It remains to be seen how much impact Kate's donning of Jenny Packham's design will have on the rest of her collections. She has quite a specific coterie of customers among the Middleton demographic already and does not necessarily need the media attention that this outing will bring.