Life, as it turns out, has been a bit of a cabaret for me of late, what with very happy New York forays to Marin Mazzie and Jason Danieley at Feinstein’s At the Regency (bliss) and then Kelli O’Hara at the Cafe Carlyle (ultrabliss). Upon my return to London just after Easter, I caught Manhattan-based Tony DeSare on his last night at the Pizza on the Park in Knightsbridge: a likable if not terribly distinctive presence who was at his best when rocking out to numbers like Johnny B. Goode.

Waddingham was something else again, not least because the late-night atmosphere at the end of an eight-performance week meant that the audience was every bit as ready to let their hair down as was Waddingham, who quickly set the tone by shedding a pair of fairly daunting heels so that she could perform unencumbered.

And what a performance! It’s the great joke of the current Trevor Nunn-directed revival of A Little Night Music, at the Garrick Theatre, that it has in Waddingham the first Desiree Armfeldt in my experience who is a full-on vocalist; indeed, the role was famously written for a non-singer in the late Glynis Johns and was superlatively inherited at the National Theatre during the 1990s by Judi Dench, who once told me (in a now often quoted remark) that she sings the way she speaks. Most poignantly, as well.

But you don’t expect Send In the Clowns to go to an erstwhile Lady of the Lake, the Spamalot mock-diva assignment that brought Waddingham an Olivier nod several years back and whose vocal swoops were much in evidence during a cabaret debut that, interval included, ran well over two hours. The sense of partying till dawn was to the delight of a crowd that included her Night Music colleagues, co-star Alexander Hanson seated front-and-center with his actress-wife, Samantha Bond, so that he could leap up to partner Waddingham as one of several very game, hugely talented guest artistes.

That lineup included Waddingham’s boyfriend (a relatively new addition to her life, one gathers), as well as various novelty acts that took the show into a quasi-surreal direction, and then, best of all, a firepower version of the great Peggy Lee number, I’m A Woman, with Waddingham joined by Anna-Jane Casey and sisters Gina and Mazz Murray, both of whom are currently on the West End (Gina in Chicago, Mazz in We Will Rock You). The ladies’ formidable lung power brought a chatty, increasingly bibulous audience immediately to its feet as, earlier, did a duet from Lakme that Waddingham performed with her mother, who has long been an ensemble member of the English National Opera.

I liked the breadth of repertoire that swung easily from Burt Bacharach to Crystal Gayle, from Faith Hill’s contribution to the film Pearl Harbor (There You’ll Be) to two numbers from erstwhile Waddingham starrer, Andrew Lloyd Webber’s The Beautiful Game, that came with a few aptly snarky comments on that composer’s over-fondness for what Waddingham termed “the money note” – a note, incidentally, that the singer has no trouble hitting dead on.

Often, musical theater performers can struggle in cabaret settings to decide upon a persona, and I felt some brief concern at the start that some unfunny, if deliberate, mangling of her surname was going to render the entire performance in quotation marks, British irony everywhere at the ready. In fact, once Waddingham opened up, she and an adoring crowd loosened up and the affection between both parties simply intensified as the show went on.

I don’t know what Waddingham will do for an encore once Night Music comes to the end of its limited run. But this much was clear on this particular Saturday night: the blonde means business.