Last Friday, the composer Cheryl Frances-Hoad lamented the stifling effects of sexism on female British composers in a veiled but powerful first-person plea. A recurring theme in her argument was this: while the sexism issue might be fraught with complexities, inconsistencies and even the occasional lurching improvement, it’s still one which needs ‘banging on about’ until artists of a particular gender have largely stopped being patronized and overlooked.

Later that day, I found myself at a screening of the remarkable film No Land’s Song at the Human Rights Watch Film Festival in London. You get a good idea of the powerful intent of this film from its context: in Iran, women have been forbidden from singing in public since 1979, unless they’re musically (and visually) ‘overpowered’ by at least one performing man. At her wits end with this curious, elusive and unwritten arrangement, the composer Sara Najafi decided to mount a concert in 2013 in which professional singers of both sexes from Iran (and France) would sing together on stage – as soloists, and as equals embedded in an sort of fusion instrumental ensemble – in songs keening with pain and protest.

Professional Iranian singers? Yep. Tehran has an active conservatory that has produced a steady stream of composers schooled in musical traditions Persian and Western – Najafi included. Iran’s bizarre decree goes so far as to suggest female teachers at the Tehran Conservatory of Music avoid singing in lessons where male students are present; the logic being that a verbal description of vocal technique should be entirely sufficient while neatly bypassing the indignity of a woman having to express herself through her voice in front of a man.

Some of the most saddening sequences in No Land’s Song – written and filmed by Najafi’s brother Ayat – occur when the sharp, bright images of bustling contemporary Tehran cut into black-and-white, mid-century footage of women singing uninhibited in the city’s nightclubs, cocktails in hand (among them the forthright Qamar-ol-Moluk Vaziri, who performed for men in Iran, without a hijab, as early as 1924).

How can a country where the human voice has been audible for centuries, and in both sacred and secular contexts, slip backwards so fundamentally? It seemed to bemuse even the quasi-apologetic government officials whom Najafi relentlessly visited, lobbied, quietly persuaded and eventually (when permission for her concert was suddenly rescinded) dismissed and declared herself independent of.

They buckled, and eventually the concert went ahead. Some of the singers involved had been ‘great names’ on Iran’s music scene before the 1979 revolution. The now grey-haired Parvin Namazi sat in a rehearsal room just after Najafi returned with the first batch of bad news from the Iranian authorities and sang a traditional Persian song with guitarist Sébastien Hoog, her voice rich and wondrous, bearing witness to decades of joy and strife. Unbearably moved, Hoog laid down his instrument and walked off-camera, his head held in his hands.

On the evening itself, Emel Mathlouthi, who became a poster-protestor during the Tunisian uprising in 2011, sang her signature ode to freedom, Kelmti Horra (‘My word is free’). Other songs filled with heartache were performed, approved by the authorities after Najafi called their bluff – explaining them away as historic artifacts illustrating Persian pride. In the gleaming modern auditorium (that presumably doesn’t see much non-male action), the audience of men and women, boys and girls, stood and sang some of the more rousing patriotic songs with the performers on stage. A history lesson this most certainly was not.

What it was, though, was a one-off. Iran’s rules on female performance remain, even after the (more) moderate government was elected in 2013. But the government position, though obstructive, was to some extent benign. ‘We didn’t get any reaction from the authorities [after the concert]’ said Ayat in a Q&A after the Curzon screening on Friday. ‘But the notable thing is that it hasn’t happened again. There were people inside the Ministry of Culture who wanted it to happen. But the problem was these mysterious people who didn’t want it to – people referred to as “they” – and nobody really knows who “they” are.’

What did happen, two days later, was a performance from Iranian musicians in the accepting safety of the Ritzy Cinema in Brixton – a few hundred yards from Gramophone’s HQ. The Light of Music ensemble played for an hour following the weekend’s third screening of No Land’s Song. They didn’t quite have the searing musicianship or compositional verve of Najafi’s ensemble, but the Light of Music instrumentalists and (female) singers played with a subtle nobility that stilled the room. Perhaps it’s patronizing to suggest that they seemed buoyed by the privilege of public performance, but the rhythmic rise and fall of the bead-filled ‘daf’ – a distinctive Persian drum – suggested so as it was tossed higher and higher as each piece reached its apex.

I wound up sitting next to the ensemble’s manager, Mohammed Najafi (no relation to Sara or Ayat). He has made his home in London, where professional musicians from Iran are rather more likely to have a career, despite the niche-within-a-niche status of his country’s music here. Talking to him after the performance, you realize quickly that Iran’s performances rules aren’t a ‘women-versus-men’ issue, but an ‘enlightened-women-and-men versus blinkered-women-and-men’ issue, just like Frances-Hoad’s portrait of the UK classical music scene. That might seem completely obvious, but in a culture as gender-divided as Iran’s, you need reminding.

But as Mohammed explained, Iran’s gender divisions are a lot less stark when you’re not in the entertainment industry. ‘70% of university graduates in Iran are women’, he told me. ‘These are educated people who are not going to be quiet about their beliefs’. So is the view of Iran’s music scene from London a hopeful one? ‘Things will get worse before they get better, but the overwhelming tide of public opinion is reason to be optimistic’ he says. And his suited, respectful ensemble plunged into a joyous encore.