Those of us who are regulars at the Royal Albert Hall know how difficult it can be for an orchestra and choir to really fill that enormous space with sound. The Really Big Chorus have a simple recipe for this: start with a choir with 1,500 members, add some top class soloists, lightly amplified, then add an orchestra whom someone has clearly briefed with the sentence "when I say fortissimo, I mean fortissimo." Oh, and by the way, choose a choral work with the highest impact beginning and end that anyone's ever written - Carl Orff's Carmina Burana.

The idea may be simple enough, but one has to assume that the logistics of getting so many singers together (including some from several corners of the world) are out of most people's nightmares. For The Really Big Chorus, however, this was a relatively routine gig: they've been doing these for several years, and their yearly Messiah gets three times that number. (For the mathematically challenged, that's 4,500 singers). Their events are enormously fun and rewarding for those taking part; we went to last night's performance to get a spectator's eye view.

Carmina Burana is a unique work in several ways. The opening tutti of O, Fortuna is justly famous, with its huge chords, a timpani part to die for, following by a layered crescendo of which Rossini would have been proud. Watching 1,500 singers stand up ready to produce those massive chords is a sight in itself, but O, Fortuna is just the start of a fascinating cycle. Carmina Burana is the only major choral work I know that is based on mediaeval poetry, written in a mixture of Latin and other languages or the period, and thoroughly secular. The poems were found in a Benedictine monastery, and given the irreligious and/or bawdy nature of many of them, it's a miracle that the abbots didn't burn them.

Carmina Burana achieves a strange and wonderful mixture of mediaeval feel within a thoroughly modern soundscape. The other thing that makes the work unique is its sheer variety and inventiveness: nothing else Orff wrote is frequently played today, and it's as if he poured his entire creative energy into this one work. There are three main sections, framed by the two singings of the hymn to Fortune: a pastoral suite, a drinking session "In the Tavern" and a series of romantic songs "In the court of love." Even within each section, there is immense variety. The hymn to spring Veris leta facies has overtones of Gregorian chant, followed by a lovely baritone solo, sung beautifully by Paul Carey Jones, followed by a rumbustious bucolic dance. There are more dances and songs, ending in a raucous love paean to the Queen of England.

The tavern section gets progressively and hilariously more profane, as we proceed from the inebriated (baritone) scholar to our even more drunken tenor (the splendid Neil Jenkins), who sings the lament of the swan as it is roasted on the spit, to the positively paralytic Abbot of Cockaigne (the legendary land of idleness), to the accompaniment of a farting tuba. The love songs gave a showcase to the third of an excellent group of soloists, the soprano Mary Bevan, who bewitched us with a creamy voice and perfect control of line: her Stetit puella and In truntina were the equal of any melody written in opera, the phrases smooth and flowing. They also provide plenty of variety and interest for the choir, augmented by the Virginia Children's Chorus, currently touring the country.

Carmina Burana must be a challenging work for a scratch performance like this, since it goes through so many changes of pace, mood and dynamics. But any worries that this would be a step too difficult were dispelled: the performance was thoroughly competent, the only exceptions being that a couple of the pastoral "on the green" numbers dragged a little, and the male chorus weren't at the same strength level as the women (largely due to force of numbers, there being well over double the number of women). But overall, the choral sound was accurate, rich and rewarding, diction was reasonable, and having such a large number of singers gave you a wonderful feel for the way Orff achieves both crescendo and thickness of texture by splitting the choir into sections and bringing in those sections at different times.

Before the main event, we heard Vaughan Williams's overture to Aristophanes The Wasps, a strange mix of depiction of the insects intermingled with gentle English pastoral interludes, and Britten's Young Person's Guide to the Orchestra, a work that was a staple of music education when I was a child but seems to have grown inexplicably out of favour in recent years - inexplicably because it remains highly effective as a demonstration of how to achieve orchestral colour.

But those appetisers weren't really what we came for. The evening belonged to the singers, to conductor Brian Kay, who did a remarkable job of keeping everything together and in balance, and most of all to Orff. The experience of seeing Carmina Burana in this setting is poles apart from anything that can be achieved by listening to a recording. It's a unique work, and The Really Big Chorus made it into a wonderful occasion.