I have just looked and you are right - an important aspect of the Spell Songs is the visual art being painted onstage and projected behind the singers on huge screens. I assumed it would be televised. What a shame!
Here's a good review: “… one of the most sublime evenings you will ever enjoy. The magic of these superstars onstage - the art, the music, the sheer, ravishing beauty of the language... one of the greatest concerts we've ever had at Hay. Long, rapturous standing ovation. People weeping for joy”
Peter Florence, Festival Director, Hay International Literary Festival