THE extract from The Annunciation by El Greco, displayed on the cover of the programme for The Sixteen's current Choral Pilgrimage, Flight of Angels, speaks eloquently for the music it introduces. The same painting, which hangs in the Prado Museum, Madrid, can be found on the choir's accompanying CD (COR 16128), and on it the green hues special to this Greco-Hispanic religious painter catch in a dashing way the unexpected message being conveyed by the green angel to the pink-clad, blue draped Virgin.

El Greco lived from 1541 to 1614. His life thus embraced almost exactly the dates of the great Spanish composer Francisco Guerrero (1528-99) and his pupil Alonso Lobo (1555-1617). They also covered the bulk of the reign of Spain's Philip II (1527-98).

The Sixteen's annual countrywide stomp has grown by stages to take in more than 30 venues, from Oxford and Cambridge to Llandaff and Truro, Edinburgh's Greyfriars Kirk and Perth's St John's Kirk; Durham, Hull, and Carlisle, to Peterborough, Ealing, and Canterbury. In practical terms, it must be substantially taxing, when as many as three or four concerts fall in a row. One recent sequence took them from Perth to Leicester in two days, and Rochester the next day.

The concert at St James the Greater, Leicester, a striking building from the start of last century, with its exquisite, decorative Italienate arches and notable apse, and, as a consequence, a handsome, rewarding, and beneficial acoustic, exemplified as well as any Harry Christophers's gift for programming, and the irresistible professionalism and mastery with which he and the Sixteen bring dance and delight, precision, finesse and pathos to the expressive counterpoint of Renaissance music, of whatever nationality.

Guerrero, long associated with Seville Cathedral, was himself a pupil of the great Cristóbal de Morales, a former singer in the papal choir. Guerrero's Duo Seraphim (1589), which opened their concert, would alone make the journey worth while. It is a paradox: written for 12 voices, yet defined with such purity and subtlety that you might have deemed it a duet or trio. It was sung with restrained rapture: echo effects at the Sanctus, and the sublime conclusion to final Amen, enabled one to see why Christophers, who writes eloquently about them, rates these Spanish works so highly.

Two works by Lobo followed, a Kyrie where sopranos paired sublimely with tenors, and a Libera Mein which one heard exemplary plainsong from the basses offset by some exquisite floating of the "Quando coeli" from the sopranos. Guerrero's Gloria (from the Missa Surge propera) included some elegant interweaving of the voices at "Qui tollis" and superlative upper voices ("Qui sedes") - whose exciting final passage Christophers drove to a dramatic finale.

Guerrero lived to see, and learn from, the first whispers of the Monteverdian secunda prattica style, and it was striking how Psalms 148 and 150 - Laudate Dominum (eight parts, dating from the 1590s) - seemed, even without a flurry of instruments, to anticipate, or be so quick to absorb, the new manner, with its vital rhythms and arresting alternations of large and medium forces.

Apart from an unexpected modern feel, like a foretaste of Arvo Pärt (near the start, for instance, at "emerunt aromata", Guerrero's description of Mary Magdalene bringing spices to Jesus's tomb positively gleamed, with magical diminuendi and crescendi that Christophers astutely emphasised to colour "et obstupuerunt" and "Iesum quem quaeritis Nazarenum", the latter beginning and growing from a wondrous pianissimo.

Guerrero's Credo from Missa de la batalla escoutez was especially outstanding for the lovely ritenuto Christophers brought to "descendit de coelis", with the tenors later eagerly breaking out; and the fascinating, almost folk-like quality that this constantly illuminating conductor found in the passage leading up, surprisingly, to "qui locutus est per prophetas". Somehow, Guerrero's personal touch here did much to humanise the sequence, rather as Haydn used to draw on folk idioms, often unexpectedly. The low-voice plainsong resurfaced in Guerrero's Vexilla Regis, contrasted with some highly effective, and affecting, interplay between soprano and tenor.

A five part Ave Regina by Lobo sparkled, rather like a light-hearted nursery rhyme; while his Ave Maria is a tour de force of canonic writing; but Christophers gave it character, evincing rich sonorities and a serene flow, and added a glorious shine at "Sancta Maria, Mater Dei". Indeed no wonder this eloquent choir always feels in a class of its own.

Lobo's "Versa est in luctum" is, it is suggested, his most famous piece, and the prolonged fade to the close ("Parce mihi, Domine") gives some idea why. The Agnus Dei I and II from Missa Congratulamini mihi, are based on the great Franco-Flemish master Thomas Crecquillon, court singer from the days of Charles V. It reminds one of the varied qualities and moods that Guerrero, and later Lobo, introduced, and at which the Spanish school excelled: ecstasy, gaiety, melancholy, longing, repose.

No group could be better placed than The Sixteen to share with us the rich subtleties, eloquence, and vibrancy of this Iberian music, here magnificently poised and beautifully judged.

For tour dates and locations, and booking information for the 19 remaining performances of the tour, see www.thesixteen.com. National box office: phone 01904 651485.