One of the great festive gems of recent times happened almost by chance. Wallen was working with actor-playwright-director Kwame Kwei-Armah, preparing a presentation at the National Theatre for a possible Music Theatre piece. At the last minute, Kwei-Armah asked for some background music to a monologue; she sat at the piano and straight away, ‘without thinking’, played the accompaniment to ‘Peace on Earth.’

• Inside the Christmas Carol: Part One with Thomas Hewitt Jones
• Inside the Christmas Carol: Part Two with Bob Chilcott

‘A few months later,’ Wallen smiles, ‘I kept thinking about it. I’d always wanted to write a Christmas carol. So, I started work, without a commission; I chose to write something in unison, using stepwise motion in the voice and avoiding big leaps so that anyone could learn it. But I also wanted a sense of unease. I love the sound of bells and dissonances; there’s an Eb in the accompaniment while the voices sing an E, so that we have momentary clashes. I wanted to capture the unease of winter and a yearning for hope and peace.’

Wallen wrote both words and music, and it was originally a song to sing herself. Written around 2006, it would sit quietly in her Errollyn Wallen Songbook until a major choral recording was made in 2019… by the Choir of King’s College, Cambridge! It was one of the last recordings conducted by the great Stephen Cleobury.

‘The impact that recording has had,’ Wallen says, ‘is huge. In just the last couple of years, that piece has been recorded by around twenty different artists, from Carolyn Sampson to Ruby Hughes and plenty of choirs. It’s even been translated into Catalan! What thrills me is that everyone can sing it. More than once I have been approached to reharmonise it and make it “bigger”, but I think its secret is its crafted simplicity.’

The acoustic of King’s College Chapel, of course, is anything but simple. ‘It’s a very long reverb, sometimes too long for quick music. In that place, you understand why the music being performed at the time it was built featured plenty of slow-moving harmonies; the voices needed space to ring out.’

‘During my year of study in Cambridge, one day the penny dropped that this place is like a music factory. In liturgical music, composers are writing for a specific service or event; it may or may not last. That had a big impact on me as a composer, realising that some of the best music you will write is for a specific event, place and people. As a composer, you’re serving your community.’

It was a particular thrill, then, when ‘Peace on Earth’ was broadcast as part of the Festival of Nine Lessons and Carols and the TV Carols from King’s in 2022. Nearly two decades after its composition, it continues to connect – maintaining that tradition of the timeless power of good festive music.

Hymns and carols were the bedrock of Wallen’s school days. She accompanied the hymns when the teacher wasn’t there, and on Sundays was responsible for choosing the pieces. ‘So, hymns and carols are the basis of my understanding of harmony and part of my compositional DNA.’

As a small child, she remembers falling in love with ‘Once In Royal’ while singing it. It gave her that ‘intangible feeling’, which through her present compositional lens she recognises comes from the magic of its lines: ‘I think it’s the way the bass moves down while the top rises; everything moves apart. How the bass moves against the voice part is absolutely crucial in a great carol. Something can be simple yet sophisticated. Then in the words, there’s the magic of vivid pictures; it’s great storytelling.’

Wallen points out the importance of semitones, too. ‘A semitone is always an expressive thing to me,’ she continues. ‘Think about “Once in Royal”; the opening notes feature a semitone. When I’m composing, I play with ideas; but when I’m honing, I pay particular attention to these intervals. Intervals are where the emotion lies. Every note must earn its place.’

Wallen has 22 operas to her name (yes, 22), and it transpires there are useful lessons taken from writing a carol that can be applied to a large-scale concert piece…

‘I’m constantly amazed how the song form in general shapes any kind of instrumental writing,’ she explains. ‘Take my Violin Concerto; the thing people always pick out is the second movement, where I quote a Yiddish lullaby and arrange it for harp and violin. It’s very simple and it seems to be everyone’s favourite section. There’s a lot to be said for the singability of something.’

Hymns also inspired her first string quartet, written as a student. They were set an exercise of writing variations and Wallen chose a hymn tune as her subject. The takeaway lesson from a hymn? ‘I’m constantly thinking in all my music and everybody else’s: what is it that makes a piece catch your attention instantly?’

Wallen has distinct memories of hearing the Choir of King’s College sing Tavener’s ‘The Lamb’: ‘it’s an astonishing piece. The dissonances are deeply striking, but the simplicity too; he creates a sense of timelessness that makes us feel it has always existed, somehow.’

‘I don’t think it’s easy to write a carol at all: to be simple yet unique. We have such a rich tradition of carols from all over the world; I’ve recently discovered a 17th century French carol that I can’t stop playing at the piano! What I love is that you immediately know it’s French; it’s the fourths, these bare harmonies that feel slightly modal. It’s very different to an English carol, but easy to learn. I’ve already tried it with young singers and it works a treat!’

Christmas is catching. Wallen’s latest opera – workshopped the week before we speak – features carols. ‘It’s like a Christmas Noye’s Fludde, and it involves the audiences joining in with carols.’

This, despite the fact Wallen declares herself ‘not a Christmassy person… until I hear Christmas music.’ Christmas comes at a time when many a composer is finishing a deadline or travelling for festive concerts… ‘so I’m never ready for it! But it’s a glorious time of year; it’s all about people being together and loving each other.’

The end of 2024 certainly requires a big celebration chez Wallen; what a year it has been for her! Just weeks ago she received the Ivors Academy Fellowship, becoming their 28th Fellow and joining a list that includes Elton John, Bruce Springsteen and Judith Weir; earlier in the year, Wallen was announced as the latter’s successor as Master of The King’s Music.

‘I feel very blessed. It’s been a long road for me as a composer and there have been so many difficult times. But look at “Peace on Earth” – that wasn’t a commission. The impulse to write music and to share has only grown stronger with the years. I’m constantly thinking how we can spread the word of the power of music to bring us together. Nothing does that better than a carol.’