But when it does work it can bring the score into a strikingly close focus. The Iolanthe at the Festival Hall last week showed how engaging concert theatre can be if - and it's a great, resounding if - it's done with unselfconscious energy and strong, resourceful singers.
Tuesday's Iolanthe had the lot, brought together by the conductor Roger Norrington who isn't exactly known for Gilbert & Sullivan but proved that the life of a period performance specialist isn't all hair-shirts and fortepianos when he directed a superb HMS Pinafore at the South Bank last year. I thought at the time that it was a better performance than the music deserved, and I think the same about the Iolanthe: an endearingly third-rate score of interest for its Wagner parody (the invocation of the banished fairy Iolanthe from her river bed pokes fun at Rhinegold) and the patriotic bombast of the "Peers' March" but not much else. Remove the wit of the libretto and you're left with an inconsequential string of miserably thin numbers that Sullivan enthusiasts will defend to the last. Not me.
Still there is a perverse pleasure to be got from bad music done well, especially when it gets the Rolls Royce treatment of the LPO and singers like Sarah Walker, Gerald Finley and Alison Hagley. Walker hasn't the contralto depth Sullivan envisaged for his Fairy Queen, but she has regal presence, Buckingham Palace vowels, and a sense of camp that many a queen would kill for. Finley(Strephon) and Hagley (Phyllis) are an established couple, the Figaro and Susanna of last year's Glyndebourne beautifully rematched here. And Richard Suart (the Lord Chancellor) has become the sine qua non of G&S in this country. Beware inferior substitutes.
But it was the orchestra that impressed most with a relaxed, effortless elegance that made everything sound as easy as, no doubt, it was. The pressure, and the wraps, were off, and the result could almost have convinced me there was virtue in this music. Almost.
There was a time when no half- capable singer let Easter pass without a stab at a Messiah, a B Minor Mass or a Bach Passion, but times have changed. Period performance has turned these things into specialist repertoire, reserved for stylistically attuned voices in modest numbers. And most of them seem to be managed by the same organisation: an agency called Magenta Music which accordingly cleans up at Easter and Christmas, not least with its own choral festival at St John's Smith Square.
Magenta's Easter festival this year included a choir I'd never heard in concert, called Polyphony. Conducted by Stephen Layton, it comes in the familiar format of 20-or-so voices, women on the top line, male altos, and a core foundation of former Oxbridge choral scho-lars. From the name, you would expect this choir to produce an agile, clean sound, capable of making sense of complex counterpoint; and so it did in a St John Passion that wasn't as strikingly virtuosic as you'd get from, say, the Monteverdi Choir or The Sixteen but still made my Easter. With good balance, purposeful choruses and eloquent phrasing in the chorales, it was an argument for this way of doing things.
But then, these days, any considered performance of the St John Passion is necessarily an argument for how to do things. A position statement. The conductor will have decided which version to use (1724 or 1725), how many voices, whether to mix the soloists with the chorus, whether the alto soloist should be male or female ... and all before he begins to think about the choice of instruments: new or old, and, if so, how old? Bach used instruments that even in his own day were antique; and since he took the trouble to specify a different accompanimental colour for every aria, you could argue that it matters to honour his expectations and use a viola da gamba or an oboe da caccia where appropriate.
For the record, Polyphony used modern instruments but a viola da gamba for "Es ist vollbracht": a half-a-fairy compromise that wasn't perfect but worked well enough. I wasn't so happy with their female alto soloist but there were some fine voices, including the adorable, high-flying Catherine Bott (in what was, for her, rather late repertory). And although the Evangelist lies uncomfortably high for Nigel Robson, he is a wonderfully dramatic singer and never better than in the rich (mid-voice) chromatics of the famous passage where Peter weeps - a text you won't actually find in the gospel of St John, but which Bach interpolates to enrich an otherwise downbeat ending to Part I. In St John the Passion narrative isn't as well- ordered in the distribution of dramatic and contemplative episodes as it is in St Matthew. So composers who set it, like Bach, have always had to tamper more conspicuously with the text.
Someone asked me recently what had become of Louis Fremaux, Simon Rattle's predecessor at the CBSO. As if in answer, Fremaux surfaced at the Barbican on Wednesday conducting the National Youth Orchestra rather stiffly in a heavy but superb programme of Berlioz, Hindemith and Shostakovich. As always, the NYO had a vast number of players - over 150 - shoehorned on to the platform and projecting a relentless density of sound. But it was still magnificent, with richly blended brass and an intensity of concentration that was almost physical. It elevated Shosta- kovich's 12th Symphony - a Soviet flagwaver, held in less esteem than most of the others - to an unexpected dyna-mism. And with those regrettable red neckerchiefs the NYO are forced to wear, you could say they'd dressed the part. It's time some kindly sponsor bought them something better.Reuse content