Visions of a radiant world beyond this one can often be found in music. Think of Elgar’s The Dream of Gerontius, or the tinkling harps at the end of Liszt’s Faust Symphony where Faust ascends to heaven, or that huge and difficult masterpiece by Michael Tippett, The Vision of Saint Augustine.

Like those great works, Vision of a Garden – a new choral work by the distinguished British composer Richard Blackford – also conjures up places seen only in the mind. But in other respects it’s utterly different. This vision didn’t appear to a mystic or penitent on a mountaintop; it appeared in the mind of a patient so ill with Covid-19 that it seemed he might not survive. As for the garden of the title, it’s actually a real one – the Jubilee Garden at the centre of Addenbrooke’s Hospital in Cambridge, an image of which floated mysteriously into the mind of the patient lying in that very hospital.

The patient was Peter Johnstone, a 73-year-old professor of mathematics at Cambridge University, and it’s his memories of the strange dreams that invaded his unconscious during that long period that forms the heart of the new piece, along with diary entries written by the nurses who looked after him. Johnstone is one of those music-loving mathematicians, and for decades has been a member of the tenor section in various choirs, including the Bach Choir, which will be giving the world premiere of the piece next month.

Johnstone feels it was probably his love of singing that led him to contract Covid in the first place. “I was with the Bach Choir on their tour to the eastern US, and we all flew out together on March 4 [2020],” he explains. “We were due to give some concerts with an American choir, the Yale Schola Cantorum, and we had a very good rehearsal with them, led by our conductor, David Hill. But Covid was already spreading in the US, and Yale pulled the plug on the whole tour.”

Back in Cambridge, Johnstone felt fine until March 17, when he started to feel unwell. Then on March 19 he collapsed at St John’s College during dinner and was taken to hospital. “Really, from March 17 I remember absolutely nothing,” he says. “I was immersed in dreams, which were extraordinary, and show what strange things the mind does when it’s disconnected from the body. It wasn’t just one set of dreams.