Ten years on and we can now relive every second of an unrepeatable musical marathon. Celebrating the new millennium (and the 250th anniversary of Bach’s death), the Bach Cantata Pilgrimage divided the 190-odd cantatas among 50 venues in a year-long project that seems more improbable by the day. That there should be a significantly durable recorded legacy from a journey whose schedule was so uniquely exacting is doubly astonishing.

These last two volumes close the story – or not quite, because Gardiner is, somewhere in the half-light of his busy existence, writing a book about Bach. But there will be no more exotic pictures adorning those satisfying black books full of Gardiner’s weaving diary, Richard Stokes’s enlightened translations and an endless supply of musical surprises. This is a classic tale of turning adversity to advantage. Would the Pilgrimage have been improved had Deutsche Grammophon stuck with the project? Would DG have made more consistent use of classy solo singers? The answer to the first question is certainly “no”, and probably both “yes” and “no” for the second.

Let’s recall that the era of “going it alone” in recording was already, by 2000, dawning on artistic institutions as a very serious and sensible option. The only problem with Sir John Eliot Gardiner’s cantata legacy is the kind of untidiness one associates with versions of Bruckner symphonies. There are 14 cantatas from the Pilgrimage year on DG but assembled alongside a number of earlier studio recordings, confusingly, under the banner of the “Pilgrimage”, including four cantatas which – strictly speaking – didn’t appear within the time-frame of the project. Then there are another 14 works which, repeated “on tour”, double up on earlier DG performances (not counting an Advent disc from 1987). And, most mysteriously, seven cantatas, BWV29, 119, 120, 157, 195, 196 and 197, which don’t appear at all – anywhere. Finally, there are three works (BWV106, 118 and 198) from much earlier releases (arguably “sui generis” works). Oh, for some coherence and co-operation in bringing this unholy mess together!

It was through this project that Soli Deo Gloria became the chronicling arm of Gardiner and his choir and orchestra. Imaginative contractual flexibility and the enterprising Isabella, Gardiner’s creative and able producing wife, resulted in 27 releases.

The pattern of variability in the performances throughout the series is reflected in these two final volumes from Weimar to Winchester, though the wonder is the high ratio of compelling results to the less satisfying. Unsurprisingly, the most brilliant and evergreen performances come from the Monteverdi Choir; one only has to hear the range of choral execution in a compilation, “Eternal Fire” (SDG177), to appreciate these urgent, luminous and unwavering contributions. Some of the best is to be heard in Vol 18: an exuberant BWV63, and probably the most delectable sound picture for the Epiphany masterpiece BWV65 ever captured: the ritualised, spice-laden mysteries of the Magi are magnificently caught.

Having remarked on choral distinction, the ability to produce excellent period-instrument playing in so many different locales is another admirable characteristic. There are unavoidably painful moments but the “hit rate” is extraordinary when you think how few options the producer could draw upon as the tour moved inexorably towards the next location with no time to dawdle.

Of the soloists, there are undoubted moments when the results reflect the peculiar conditions of performing such profoundly demanding music on the road, exacerbated by some singers whose technical and expressive range simply cannot penetrate the heart of the music; this becomes especially marked on repeated listening. Yet, for any singer, such a gruelling schedule is hardly a recipe for reflective immersion and in the circumstances we must tolerate the occasional “al dente” performance.

There are countless highlights in the final two volumes. The duet-singing in BWV32 between Claron McFadden and Peter Harvey is memorably rewarding. Similarly cathartic is the peerless, reassuring expression of eternal union in the great duet of BWV154 with Michael Chance and James Gilchrist. Gilchrist has consistently delivered the most outstanding solo performances over the series. He also delivers one of the best BWV55s since Ernst Haefliger. Gardiner can take chances with Gilchrist he rarely takes elsewhere. In BWV115, on the other hand, the risk of a slow tempo for the soprano aria, “Bete aber”, is less successful. For all Joanne Lunn’s admirable tonal focus, sustaining a line with the necessary grip is asking a little too much. Indeed, if there is one regret in this series, it is the lack of a regular top-class soprano to lead the line, as Koopman enjoyed with Sibylla Rubens and Sandrine Piau.

Yet, as I write, Gillian Keith is gloriously assuaging in “Ich halte er mit dem lieben Gott” from BWV52. Perhaps that’s what this series does so uniquely. It tells a wonderfully unpredictable story, at the point of execution, of a corporate spirit of discovery as musicians grapple with and inhabit a repertoire of unfathomable depth. The relative control and consistency of the studio gives way to a viscerality of response, bursts of concentrated vigour and raw pathos, underpinned by endless supplies of sheer pluck.

In this regard, the Pilgrimage stands alongside the pioneering Leonhardt-Harnoncourt set: rough edges and some misfiring; but the love and character in this music find no greater advocacy than here. For this we must thank Gardiner’s Pilgrimage troupe and all those knights in shining armour who rescued the project in the dark hours before it took the millennium by storm.