The 80s live on in Cyndi Lauper, who takes to the Pyramid stage looking every inch the rock chick of yore in her silver bustier and matching trousers and platform trainers, beneath a blue blazer also attached with streams of icy-blue tulle. There are also fingerless net gloves.

It’s a big look, and Lauper has the energy to match it, making full use of the stage and making forays out towards the crowd. They’re here for nostalgia, as she acknowledges with her opening track, The Goonies ‘R’ Good Enough, her 1985 single from the much-loved film. From there it’s on to She Bop, one of Lauper’s better-known songs, though outside the handful of classics most of the audience are baking in the mid-afternoon heat to hear.

Lauper’s brief solo on a treble recorder – not an instrument you encounter often in adult life, if you’re not a parent at least – adds to the sense of whimsy that’s being built from the stage, with Lauper’s large band kitted out in equally outré 80s fashion. Into the Nightlife is an unexpected winner, outside of the big hits, sounding a bit electroclashy and even reminiscent of Fame Monster-era Lady Gaga. The rockier tune lifts the energy from the crowd, further geed by Lauper’s own vigour – she uses her tiny frame to its fullest extent, punching the air, jumping and leading a clap with such gusto her silver bustier rides up.

But Lauper’s voice, reedy even in her heyday, is not as robust as it once was, and it can be hard to pick out the melody of the less well-known inclusions of her setlist. The playful interplay between Lauper and her male backup singer on Rocking Chair is fun to watch, but the song is messy and hard to follow, undercutting Lauper’s blazing stare down the barrel of the camera in its conclusion.

There may be some sound issues at play, it seems, from Lauper’s brief exchange with a team member. We’re back on track with I Drove All Night, which allows Lauper to show off her more potent mid-range vocal at close to full throttle. The crowd is willing her to make the high notes, and she does, holding them to resonant effect as the song crashes away behind her.

But the rest of the set is a bit stop-start in the same way. Through her songs Lauper doesn’t stop boogying, at points reaching a Muppets-level freneticism. But her interactions with the crowd, and storytelling around her songs and legacy, is more diffuse. Lauper introduces Time After Time by saying she sang it on Top of the Pops, underselling perhaps the best of her best-known songs (which she co-wrote). On the verses she doesn’t quite bottle the magic; the rhythm gets away from her so that it sounds meandering. The crowd is of course eager to help her out on the chorus, helping to recover some of the power that’s been lost from her distinctive quaver.

Her cover of the Brains’ Money Changes Everything again seeks to inject whimsy with a melodica solo, but everyone – not just the girls – knows exactly what fun they’re here to have. Lauper begins her defining hit a little wonkily, and though her voice is still girlish it’s lacking the youthful defiance that makes that song so irrepressible. But Lauper’s obvious enjoyment in the crowd’s response is charming, and her explanation of its impact and how she chose to channel it into activism (today’s cause is the White Ribbon Alliance) gives heft to what might easily be dismissed as a novelty record. Lauper’s rapped breakdown “girls just want to have fundamental rights” draws a big whoop from the crowd, even if she elaborates with broad-brush strokes: “It’s time the world understands that women are half the population of the world, and we deserve to be treated equally no matter where we are from!”

Perhaps unsurprisingly, there’s a sizeable exodus after Girls Just Wanna Have Fun as the crowd, beaten down by the heat, pursue shade and refreshment over sticking around for True Colors. Like Time After Time, it struggles to emerge against the muddy sound and some pitch issues on Lauper’s part, perhaps exacerbated by the outdoor venue.

With this set (and her recent night at the Royal Albert Hall) a curtain-raiser of sorts for her forthcoming farewell tour, her choice of closing song – I’m Gonna be Strong, a cover of Frankie Laine cover popularised by Gene Pitney – seems a little trepidatious.