Just as Blake's poetry responded to the social issues of his day, Westbrook's settings reflect the changing relationship between the arts, culture and politics from the Sixties to the Nineties. Agit-prop and Communist Party- inspired radical interventions; the waxing and waning of the CND movement; even, perhaps, the reawakening of a new spiritual consciousness - all have come and gone in the life of this extraordinary work.
First commissioned for Tyger, Adrian Mitchell's musical play about Blake at the National Theatre in 1971, the settings have had a long and eventful career since. A number entered the repertoire of the Westbrook Brass Band in the mid-Seventies and were recorded in 1975 on the album For the Record. More were added for Glad Day, the 1977 television play by Mitchell that marked the 150th anniversary of Blake's death. "Holy Thursday" from Songs of Experience was first arranged for The Orchestra - a collaboration between the Brass Band, the radical rock group Henry Cow, and the folk singer Frankie Armstrong in the mid-Seventies. In 1980, The Westbrook Blake was released as an album. In 1982, at the insistence of the anti-nuclear campaigner EP Thompson, Westbrook recorded "The Human Abstract" as a single for CND. For a revival of the Blake repertoire in 1996, Westbrook added the opening instrumental piece that gives its title to the new collection.
This genealogy is important, partly because the project has meant different things at different times, and partly because it is so very, very good. The combination of Blake's visionary verse and Westbrook's often sublime music, complete with spirited jazz solos and the voices of both Kate Westbrook and the almost unbelievably intense Phil Minton, is a very strong brew. At performances, people are frequently moved to tears, and not entirely because of the beauty of the words and the music. They also cry, you suspect, because the sequence has come to represent a kind of final wail from a lost radical and humanitarian tradition. And as socialism gets increasingly hard to find in politics, its presence in art becomes all the more precious.
Westbrook once told me: "In a way the jazz group is a model of the perfect society, with each individual contributing what they do best, without individuality being suppressed. In a way, it is a model of socialism."
The burden of the response the Blake settings evoke is something Mike Westbrook bears rather heavily, and probably the reason why he prematurely retired the repertoire for a number of years before the 1996 revival. "It belongs to everyone. I'd always rather direct people's attention to our current work, but it's been important for a group of musicians and audiences who find it touches something deep inside. It's to do with Blake's vision, and also our spiritual feelings and our need for that kind of release.Even if I don't write that sort of music any more, the Blake material has all these grand emotional themes that you're lucky if you find once in a lifetime."
Ironically, while the composer has moved on, in many ways society hasn't. At the time of the premiere of Tyger in 1971, could there have been so many Blakeian lost souls wandering homeless in the London streets or sleeping in doorways? "Both on the political and social fronts, the songs still seem relevant," Westbrook says. "The dilemmas and the evils of society are still there, as is this wonderful vision of the redemption of the world."
Westbrook is now 62 and one of the world's greatest jazz composers. His career goes back to 1958, when he formed his first band in Plymouth before moving to London in 1962, and encompasses complex large-scale works such as The Cortege, On Duke's Birthday, and London Bridge is Broken Down. Without subsidy, he manages to continue to command the services of a regular big band, many of whose players have remained with him over three decades. But, perhaps gallingly for a composer of such reach, the favourite work for many of his audience still remains the relatively simplistic Blake settings.
"I don't feel it's a millstone now; the balance has shifted," Westbrook says. "It still feels relevant, it's still needed, and I don't feel that anyone else is doing this kind of work. I'm very involved with other projects; with Blake I'm just going at it." The problem, if there is one, is perhaps to do with Blake himself. "He was a tremendously sophisticated artist, but with an innocent, childlike view," Westbrook says. "The music is wonderful and inspiring, and it brings out something in people, but it is conceived in fairly simple terms. You can't just go for the highest emotional impact all the time. This is what pop music does: it works away like water on a stone on the most susceptible emotional areas."
One of the greatest pleasures of the new album is the opportunity it brings to once again hear the remarkable vocalist Phil Minton. A colleague of Westbrook's since the group Solid Gold Cadillac in the late Sixties, and an ever-present in the Brass Band, Minton is the most thrilling singer we have, with a messianic passion so fierce that his performances glow as brightly as Blake's own tyger. "It's probably the stuff we did with Phil that stands for all time," Westbrook says.
In the Westbrook Blake, you may not know the tunes or the words, but you'll certainly recognise the place-names. When Phil Minton sings (in "The Fields") of "The fields from Islington to Marylebone, to Primrose Hill and St John's Wood", you even find yourself thinking of post codes and bus routes. When he gets to the bit about building Jerusalem there, don't choke back that sob. You never know, it might still be possible.
The Mike Westbrook Brass Band & the Senior Girls Choir of Blackheath Conservatoire of Music will perform 'Glad Day' at the Purcell Room, 7 June (0171-960 4242). 'Glad Day', (Enja) is released on 14 June