Opera: SIMON BOCCANEGRA 1857 version, QEH South Bank / 1881 version, Royal Opera House, Covent Garden

Simon Boccanegra, before and after. Now this is where the Royal Opera's Verdi Festival comes into its own. In the past week, we've been afforded an instructive insight into Verdi's development as a composer in the 24 years between the original Boccanegra (1857) and its radical revision of 1881. And what might at first have appeared like minor tweakings of the score, a re-welding of a transition here, an extension of material there, have - in performance - proved quite startling.

Boccanegra was already something of a mould-breaker back in the 1850s. The austerity, the dark and forbidding tinta of the score with its predominance of black-note configuration in the melodies and its broody oceanic undertow, violas and cellos dictating the string-band colour, the lower registers of the clarinet and bass clarinet, horns and trombones working like Paolo's poison into the very fabric of the sound.

With the period instruments of the Orchestra of the Age of Enlightenment under Mark Elder, last Thursday's concert performance at the Queen Elizabeth Hall pulled focus on these characteristics with an almost ruthless immediacy. The sound may have been leaner and rowdier, more primary, the brassy trumpet and piccolo-topped tuttis tending to brighten the complexion of key ensembles, but its sinew and rhythmic profile were vividly realised. I love the gut- string sound, trenchant where needs be, but warmer and stringier, too, and what a presence those louring period trombones can exert. For that matter, what a fine, theatrical Verdi conductor Mark Elder is.

There is, of course, the little matter of the revised finale to Act 1 - the great Council Chamber Scene. It is quite staggering how the insertion of this one scene completely turns the opera around, motivating (through the final curse) the villain of the piece, Paolo Albiani, and elevating Amelia Grimaldi to a compassionate and very real heroine. On Tuesday at Covent Garden, Bernard Haitink rose to it with a commitment and nobility that were both thrilling and gratifying.

It is here, of course, that the soprano gets to sing one of the loveliest phrases in all Verdi (and don't we just miss it in the 1857 original), the kernel of one of his finest ensembles, a dramatic foreshadowing of Otello Act 3. Haitink had the American Renee Fleming in her first Amelia, showing - for those with ears to hear - a thing or two about the Verdi style. Every phrase is personal. Her very first brought elegant hairpin sculptings of the line, and from the climax, a long and grateful diminuendo, taken away on the breath and then lovingly dovetailed into the next phrase. This is a wonderful talent - a complete voice (creamy and free throughout the compass) and a genuinely musical imagination.

To hear Fleming in relatively close proximity to the great white British hope, Amanda Roocroft, was revealing. In Elder's concert performances, Roocroft was recovering from an infection, but still managed plenty of voice. It continues to fill out, not the loveliest sound but one of substance. It's the artistry, though, that I miss. I hear a well-schooled, textbook singer (though, unlike Fleming, one inclined to short-change us in her phrasing), but nothing as yet that is her very own.

Roocroft was partnered here by the Argentinian tenor Jose Cura, dark and individual in timbre (in that respect, one up on Michael Sylvester at Covent Garden), but for my money lacking resonance, both in sound and style. The Boccanegra was Anthony Michaels-Moore, a fine singer, but no match for the vocal and physical authority of the Royal Opera House's Alexandru Agache.

Elijah Moshinsky's statuesque but empty staging adds little that we could not imagine at the QEH. Close your eyes: the final encounter between Fiesco and Boccanegra, the ache of Verdi's violas in the closing moments - these are the sombre sounds and sights of Simon Boccanegra.

n In repertoire to 21 July at the Royal Opera, Covent Gdn, London WC2 (0171-304 4000)