What on earth – I wondered when I first heard 2004’s Milk-Eyed Mender – WAS this combo of harp plinking and faux-naïf girlishness: a voice whose apparently affected Melanie-meets-McGarrigles timbre has proved to be archetypal musical Marmite? I’ve always been partial to the Kook brigade – a chain of fearless females stretching from Nyro to Bjork via Bush, Mary Margaret O’Hara and more – but not even they quite danced on this high wire.

I heard the voice – was it on one of Peel’s last shows or on the late Robert Sandall’s Mixing It? – and straightway embraced it as, yes, mannered but, no, not precious or twee as Newsom’s detractors would claim it is. I heard it as real, true to its own emotional shtick, fully engaged and immersed in the girl’s strange and gorgeous melodies.

What could we call the style that then evolved on 2006’s Ys [“Eeese”] to become a tingling hybrid of Bjork’s Vespertine and Van Dyke Parks’ Song Cycle? “Chamber Americana”, maybe? The story goes that Newsom’s then-squeeze Bill (Smog) Callahan turned her on to Song Cycle; that Parks was then hired to write string and woodwind arrangements for such outré Ys pieces as the closing ‘Cosmia’. Some deemed Parks’ micro-baroque decoration excessive, but to me it exquisitely complemented the Mills College graduate’s sonic filigree.

For the closing Sunday set on End Of The Road’s Woods stage, Newsom is accompanied by, among other things, violins – played by a pair of brilliant women who play all kinds of other obscure instruments as well. The two equally brilliant male players also frequently switch to other unusual instruments, though they additionally supply the more conventional rock elements of drum kit and electric guitar. (I suppose the point is that this isn’t rock music at all. I’m standing in the dark Larmer Tree drizzle because Newsom has so beautifully broken free of rock’s terminal tropes – and also of the many “alternative” music forms that have become their own generic traps and cul de sacs.) I’m confident she will astonish me, even if she plays the rolling and relatively orthodox piano pop of Have One On Me‘s heavenly ‘Good Intentions Paving Co.’.

The latter turns out to be the last song of a breathtaking set characterised by intense concentration: by dazzling beauty and complexity, high drama and virtuoso passages more thrilling than any pounding rock discharge. She’s a strange sprite, is Newsom: part faerie queene, part California gal with her beaming smile and her “thank you so MUCH” after every round of applause. But there’s nothing fey or ethereal about her singing or her playing – mainly at the harp that sits centre-stage, sometimes at the grand piano at the back (for ‘Good Intentions’, ‘Waltz of the 101st Lightborne’ and more). Every interlocking part of her songs is so precise and considered, every long knotty vocal line unspooled with such passionate conviction.

It’s not like I don’t get how odd Newsom’s voice is: it’s extreme and no mistaking. But it’s come a long way from the ditsy Appalachiana of Milk-Eyed Mender: it’s the human voice as theatre, a sound as rich and beguiling as any singing I know, whether it’s tackling the convoluted swirls of ‘Cosmia’, the crazed nursery rhymes of ‘Monkey and Bear’, or the slowly unfurling ecstasy of Divers’ ‘Time, as a Symptom’.

Maybe it’s just because I’m growing old and slowing down that the work of women like Bjork and Newsom feels so much more fluid, intuitive, and exuberant than the constrained music of their male counterparts. All I can say is that Newsom – kooky or otherwise – possesses a talent that’s unfettered but never pretentious. She’s a poet and songstress whose voice soars and carries me into the lowering English sky: into the cosmic mystery that all the greatest music intimates.