Live: Stage Whisper. Charlotte Gainsbourg with Connan Mockasin, Somerset House.

On its day, its stately surrounds elevate Somerset House up among the finest of London's musical establishments; up amongst the Roundhouses and 100 Clubs of the capital. They are, however, only employed as such for a few days over a week of each year and tonight it bids its final farewell to 2012. Furthermore, we're at the mercy of the elements out here with shows taking place come the rain, sporadic shine or inevitably diluvial thunderstorms to plague our ghastly BST. Such downpours held off on Monday as Susanne Sundfør cast a spellbindingly revelatory darkness over the courtyard and, mercifully, tonight meteorologically typifies a much thirsted for summer sky. Cloudless, if pigmented with the the city's starless, yet inextricable lightness; pollution and all that preposterous detriment. This is to be its day.

Before turning undivided attentions to Charlotte Gainsbourg and Connan Mockasin's inexpressibly supreme re-orchestrating of Stage Whisper, I ought to confess to an undying affection for the former: having enchanted via the media of screen and song, she's a personal heroine and as such I'll undoubtedly swiftly exhaust every superlative. For it'd take as many to fuel our sheltering sun as it would to illuminate the expressive irradiance of Gainsbourg with word alone. She's a marvel, and through retrospection achieves an unerring cultural relevance today without ever even so much as intimating a submission to selling out or some such. Putting the independent back in indie, everything appears as though it's executed according to her own personal terms and consequently, well, that Antichrist imagery of genitalia mutilation aside when did she last commit a career misdemeanour? Not one instance gambols to mind.

And as for Mockasin? By gum, he's come a heck of a way. This sort of breezy; blustery; gorgeously watery open air spectacular must've seemed implausible to both we and he just a few short years ago as he was teasing us with almost unlistenably avant-garde, woozy psych pop. The deranged scat of Sneaky Dog Friend is but a distant whiff of history now, and Hosford has begun to resemble a young Richard Branson to all superficially aesthetic purposes. A similar fate has befallen Sam Eastgate: for all the rumination on whatever became of Late Of The Pier, few would surely have assumed that the then objectionable frontman would go on to muscle in on this malformed vanguard of intricate artistry and yet here he is, effectuating synthetic sax solos to the debonair funk nostalgique of the Serge-scribed Ouvertures Éclair. The song suave as myriad Parisian businessmen, that a française enchanteuse, an emigrated Kiwi and his English Oompa Loompa-like backing band may converge artistically and publicly slap bang in the centre of London speaks volumes of the positivity to be derived from multiculturalism.

Therefore despite the clashing curls of respective mother tongues (Gainsbourg chirps and captivates in both French and English. Mockasin is, meanwhile, largely unintelligible for the most part) the pair seem to speak the universal language of eclectic harmony. They're reading from the same sleeve notes; tuned into the same frequencies; eruditely versed in emotivity and the result is as sonically multifarious as it is multicultural, the pair flittering from chanson française, to funk, to soul, to psychedelia and over to sole-corroding, incendiary Paradisco. Antithetical AmEx ad propaganda meanwhile (this is the second year the credit card behemoth has sponsored the annual Summer Series), splashed across side of stage hoarding reads: 'London illuminated: Putting the spotlight on music'. Quite what the financial bigwigs have to do with this form of entertainment is enigmatical to say the very least, although just as this neo-classical expanse is greatly improved when illumined as it is tonight, seeing Charlotte stage-centre and spotlighted is enough to enlighten even the glummest midsummer visage.

If the clientele here conglomerated may differ a little to that of Monday, that most are of la même nationalité is of negligible surprise although an adorable Science Of Sleep chic predominates aesthetically. Alcohol, Coffee and Cigarettes all consequently omnipresents, Gainsbourg's actor status – needless to say a foundational element of her character composition – appears to have become almost inextricable from individual, and yet tonight any dramaturgy is dumped in favour of doe-eyed nonchalance. As she laments on a scintillating, vaguely Nouvelle Vague-esque Memoir, "I might as well be anyone at all" we are but thankful that we've such a lucidly loveable personage to fall helplessly at the wedged heels of. Indeed with her father's features she couldn't possibly have attracted anything but indie stardom – undesired or otherwise. I, for one, am delighted she complied with her vapid, quintessentially French rock chick charms – and every off note or mistimed gasp only gently imperfects her seraphic allure. She's human after all, and thank fuck she's here.

However prior to being here tonight, the prospect of Mockasin mucking in rather than conventionally supporting seemed misconceived and yet when his interlude of sorts arrives, as his lustrous sounds coincide with the pair's artistic vision everything makes sense with a crystalline kinda clarity. Liquefied slow jams and dolphin jives friendly not only to humans and all species of tuna but also to every thing bestowed with functioning lugholes, although presented by Gainsbourg as not needing "an introduction. But he's Connan Mockasin" he proves as indispensable to this resplendent unravelling of an evening as she. The gloriously languorous It's Choade My Dear – with Gainsbourg on appositely lackadaisical drums and glaringly spotlit – warmly reacquaints with the dribbly joys of Forever Dolphin Love, whilst the title track is so adorably loose that, personified, no ceinture could keep its slacks hoisted. The former is punctuated by impassioned cries of "Take it off!" which rebound off chiselled walls, enclaves and columns – of course as well as Gainsbourg's whispering cymbals – whilst the latter is propelled by ebbing waves of moistly translucent guitar which ooze from Mockasin's callously maimed Strat.

During this intermediary phase Gainsbourg casually saunters the boards, plinking odd bits and mellifluous pieces: a ding of xylophone here; a beady clack of maraca there. If anything her contribution is a little too blasé for comfort, only momentarily dipping her toes into the thalassic beauty which is by now in full flow. Indeed were one to compose an orchestra purely of coral reef debris it may not sound quite as aquatic as a Mockasin-inspired rework of Bowie's Ashes To Ashes. And at this point the parallel sketches itself: for with The Thin White Duke all but unofficially retired, there's nobody better to take up the outré mantle than Hosford. His fluid songwriting style which flouts convention and verse-chorus constitutions mimics to a not insubstantial degree that for which David Robert Jones became notoriously famed and although the track itself may mark the only momentary tedium, it alleviates the bewilderment Mockasin visibly engenders a minute. "Thank you for being patient", he proffers deferentially as it irrefutably seems as though that introduction were requisite after all. Hopefully somebody, somewhere in this modest crowd felt the 'Dolphin Love and got enlightened tonight...

Thus although Mockasin's slinky psych funk may befit Gainsbourg's insouciant exhalations like a particularly clingy Jean Paul Gaultier getup it's she, stooled and sassy in a churchly shade of white whose purity and perfection veritably gleam: when doused in her own torrent of nonchalance on All The Rain; or on the shiny Greenwich Mean Time – perhaps never performed nearer to that particular Royal Borough than today – which is drowned in a radical deluge of phase of Mockasin's making, sounding as an inside out fish tank may; amid the tumbledown acoustics of Heaven Can Wait. She's a national trésor and was inevitably endowed at birth with the nicest of voices, both spoken and sung. And the more disenchanted and demure of manner she may seem, the more she enchants. Thus if she is in fact putting up a punctiliously rehearsed façade then mon dieu! it's worked wonders. For she appears as gawky as I could've only dreamt in effortlessly exuding the captivating whispered recitals of Jamais: one hand in her pocket and t'other clasping the microphone as though it were the splintering pencil of an anxious étudiant about to enter a very first French oral never more to be recalled, it's all pure wonderment. And with Mockasin relegated to peripheral xylophone duties as she soothes: "The performance of a lifetime/ It's my only starring role", conviction is absolutely key and it takes next to no persuasion to go with this evocative lyrical guile.

The Songs That We Sing is palpably Air-y, although that's scarcely surprising with the Frenchmen and woman having collaborated quite so extensively on 5:55 whilst Got To Let Go, tonight transformed into honky-tonk joy, is vastly intensified – and with it enhanced almost impossibly – by Mockasin stepping in and strutting all over the once recorded vocals of Charlie Fink. It's a superlative song from a character who has increasingly become contemptible to the point of the irredeemable – at least musically – and marks the first of only a few moments whereby the pair collaborate fully. They crouch, Mockasin a mischievous imp blurting incoherent yet overjoyed garble before the song takes a more sinister, if surreal turn as they chant in melodious synchrony: "It's a deadly revolver to your head", only then arising to their feet. This repeated statement of human brutality, when set against such placid soundtrack, again elucidates how well contrasts can combine to amass a solution incalculably superior to the sum. And this is disproportionately amazing stuff.

This is surely what Big Deal were bargaining for, as at points we're made to feel the dejected parents who peer through the almost entirely slammed bedroom door of your typical (or rather typically British) reticent adolescent. Except beyond plasterboard/ pine we see childlike figures serenading one another, one showing off newly acquired six string skills to the prepossessing although ultimately, drastically intangible girl next door and this never quite requited lust is never better exhibited than on the ambling frolic of Me And Jane Doe.

Despite coming together on Stage Whisper collab Out Of Touch, this creative starting point is foregone rather fascinatingly and come the premature yet inexorable end to this absolutely magical soirée I long for one more encore; one more chanson. That one would undoubtedly do. Although irregardless, they make for a seamless match: Gainsbourg is the Bardot to Mockasin's, um, Gainsbourg; the Nico to his Lou and never more so than on another sublime paean penned by Daddy, Don't Forget To Forget Me. Poor parenting advice perhaps, although a stunning close to this, the main segment of show.

And as a show composed of a measured blend of old, new and never hers, we're resultantly never bored. It has been the paragon of musical performance, and I'm now out of superlative. At this point I apologise for the unending veneration, but Gainsbourg tonight became a bona fide idol and irrespective of however involuntarily such status may have been accomplished, how she could ever have proven to be quite this wonderful persists to perplex.