Despite his reputation as a womaniser, it was deep-seated insecurity that directed Gainsbourg towards his dandy-esque fashioning. Growing up during the Occupation to Russian-Jewish parents, 12-year-old Gainsbourg had been forced to wear a star on his sleeve. While as a child he would pretend the star was that of a “sheriff or big chief” as he later sang, as an adult it was less easy to ignore press which focused antisemitically on his appearance.

“He had a lot of neurosis about the way he looked,” Kosmos says. “He thought he was ugly. So, his mentality was, ‘If I look like a Hollywood star, then I can be attractive.’ It was a suit of armour for him.”

Gainsbourg’s first fashion era, when he was starting out and mainly playing piano in clubs, consists of simple suits, aping jazz musicians he idolised such as Mr Boris Vian. But once he got money, everything changed. In the mid-1960s, Gainsbourg was instrumental in the yé-yé movement, which saw him travelling over to Swinging London to record rock tracks. These trips became even more regular once he started dating Birkin, who lived just off King’s Road, then the epicentre of European cool. This was reflected in his wardrobe choices: double-breasted trench coats worn with the collar pulled up insouciantly and fitted, mod-style suiting.

Gainsbourg was said to have chosen recording studios based on which tailors could be found nearby. However, the singer’s preferred location to buy suits was Paris, and specifically Renoma, a new brand outfitting French youths in “le style anglaise”. Gainsbourg modelled for the tailor and it was a constant source for suiting, including his pinstripe suits that he wore well into the 1970s, later with denim or khaki military shirts. While Renoma is little known today, both Messrs Karl Lagerfeld and Yves Saint Laurent shopped for suits there, too, and its founder once accused Saint Laurent of stealing his silhouette.

Birkin was another key style influence on Gainsbourg, and much of their fashion trajectory can be tracked in tandem. Together they were brand ambassadors for Lee Cooper denim, they modelled for Cerruti as well as in British Vogue in matching looks, and Birkin’s associations with Lanvin led her partner to wear one of their suits to market his album Histoire de Melody Nelson.

As Mr Jeremy Allen writes in Relax Baby Be Cool: The Artistry And Audacity Of Serge Gainsbourg, Birkin also “helped to bring out Serge’s feminine side” giving him bracelets and diamond necklaces, complementing a watch collection that included a Rolex Daytona, a Breitling with a custom strap and an Omega Speedmaster. She did, however, maintain he stopped shaving: “Otherwise he looked too shiny… It made him look too young,” she told Allen.

The singer’s most notorious costume, however, was created after he and Birkin broke up. “Gainsbarre” was an alter-ego, a “mask” on whom he could blame antics such as aggressively propositioning Ms Whitney Houston and burning a 500 franc note, both on live TV. This grey-haired, generally inebriated version of Gainsbourg more frequently wore dark sunglasses and his clothing, while still typically “Serge”, moved from artfully effortless to salaciously dishevelled.

Gainsbourg’s house might be the grandest gesture of his aesthetic intentions, reflecting his sartorial aspirations inwardly. Its dark interiors are a cabinet of curiosities filled with everything from first-edition books and marionettes to empty Cartier boxes and reams of pearls. It is here the spirit of Gainsbourg as a poet, “our Apollinaire… our Baudelaire,” as President François Mitterrand called him, is most alive. It is also here that, when being taken to hospital in 1973, Gainsbourg insisted on bringing his Hermès throw. Jane may have had her handbag, but Gainsbourg had his blanket.