And it gives me a chance to brave the intimidating jargon of the organ world. What we have here is a three-manual tracker-action, with 48 stops, 3,574 pipes and 999 stop combinations, commissioned by Simon Preston from Johannes Klais of Bonn, the Mercedes Benz of organ builders. Cost, including structural alterations to the west gallery, pounds 850,000.
'May I look inside?' I ask, like Bluebeard's next wife. Christoph Linde, head voicer from Klais, opens a panel behind the console. We enter the main chamber, big as a king-sized bed.
All is neat, symmetrical, a carpenter's paradise. Solid European white oak makes the main frame and wind-chests; wafer- thin levers of Alaskan pine connect the keyboard to pipes on the chests, much as the toe bone is connected to the collar bone, I suppose. Christoph commands an apprentice. 'Andreas, ein Register auf dem Hauptwerk - Prinzipal Acht]'
Bonk, bonk, bonk . . . Alaskan pine jigs up and down. (This is a relief. My handbag had clouted several wafer-thin levers when I turned round.) 'It's very dense and stable,' Christoph says. 'Otherwise the keys would stick when the heating comes on.'
He plucks a gleaming pipe from the wind-chest - it sits freely in its hole, held steady by its own weight. The litany begins. 'A large number of metal pipes are made from tin and lead, the alloy ranging from 80 per cent tin and 20 per cent lead to 80 per cent lead and 20 per cent tin. The principal choruses which make the tonal backbone are almost pure tin; but for the flutes and the soft, broad, warm-sounding stops the amount of lead increases . . .'
Some pipes wear hairpins; some have ears and hats around the mouth, flaps on the top, bulges at the waist. Some bend to fit an awkward space - it doesn't affect the sound. That's enough] How do you actually design an organ? But there are trade secrets here, and half a million pounds hangs on each contract won; so it's not surprising that Christoph gives little away. He persuades me that, even today, an organ- builder can walk into a building, look at the ceiling and get a feeling. 'There are certain rules of thumb about acoustics and measurements of the building, but basically you do it by experience and consideration of the reverberation. Here we are very lucky because it's 2.7 - far better for an organ than the dry concert halls they build for over 2,000 people nowadays.'
Looking at these ranks of pipes, tightly packed yet with enough room for each to speak, I wonder how they decided where to put everything. Klais's ingenuity in fitting 48 stops into an 18th-century English case played an important part in winning this contract. The case - discovered in a Suffolk church and donated by a local grandee who had been married in St John's before its wartime bombing - is perfect architecturally; but in its original state it would have housed only a quarter of the stops required for a modern concert organ.
'The organ was set up in the shop in Bonn before it came to London,' Christoph says. 'The console, wind-chests, wind-trunks and many pipes. But that's only temporary because voicing and tuning have to be done on site, in the right acoustic.'
But if the whole box of tricks is ready- made, like something from Ikea, how can you alter what it sounds like on site? 'When these pipes are prepared in the pipe shop, they don't sound at all. Voicing is when you work on the mouth of the pipe to seek the right tone and character. Tuning is bringing it to its proper physical length.'
Christoph's own voice indicates that voicing is artistic - delicately fiddling with the upper and lower lip of the pipe to adjust the column of air vibrating inside. Tuning is mechanical, boring - perhaps no more than the gentle bashing of the pipe-end inwards (which sharpens the sound) or outwards (which flattens it). How many pipes can he voice in a day? Maybe 200. 'But then you can spend four hours working on one reed pipe.'
Today he is tuning the Plein Jeu in the Swell. May I see? Up we go, Bluebeard and I. Up a shaky step-ladder. And up further, vertically, hanging on to a stout mahogany 16' Bourdon. On the way I appreciate an unorthodox design feature. The largest pedal pipes are built inside the floor of the gallery, and speak through openings like central heating ducts. Anyone standing nearby vibrates with them, and a 32' Untersatz blowing up your skirt is quite thrilling.
The Swell is no place to party. (Swell pipes must be enclosed in a box with louvres which open and close to vary loudness and tone.) At St John's, Klais has craftily tucked it right up at the top of the organ - a kind of attic conversion.
Here lie Christoph's tools. No graphs or meters; just a few brass hammers, mallets, pliers. Andreas plays an A on the Plein Jeu. I shudder with pain: it's the aural equivalent of a mirror reflecting the sun into your eyes. 'Und mit zwei Fuss]' A flight of arrows arrives. In the name of Saint Sebastian, I wonder how Christoph, whose skill demands exceptional hearing, can endure this enormous din for hours at a time. He produces the yellow foam earplugs he normally wears for tuning mixtures.
Simon Preston bounces in. How much does he understand about all this? 'Nothing at all, when I started,' he says gaily. 'But organ-builders will talk about these things until the cows come home.'
Whereas some people have had children, Simon has had organs. 'Yes, I have rather littered them about. But I don't believe in writing books about it, I think you have to get the organs built.'
Both cathedrals where Simon has distinguished himself as organist bear witness. First, the Rieger at Christ Church, Oxford - a splendid Common Room scandal, especially when the first foreign builder contracted went bankrupt. Then the Harrison and Harrison revamp which blew the dust off Westminster Abbey - after Simon left, the Dean put a padlock on the Bombard. Simon likes an organ to have character, teeth.
His task as consultant for St John's involved far more than devising a list of stops. He spent many hours with Klais in Bonn, including two days pounding a dummy keyboard to design the console. 'Too many consoles are like Japanese dinners,' he says. 'Absolute hell for the first five minutes, until you get used to sitting there with your legs trussed underneath you. I wanted this organ to be comfortable for people who may only have a short time to practise. And the stops at eye level, all the pistons visible, numbered so you can see what you're changing.'
Simon puts his music on the desk. Virgin clean. Not a single phrase, fingering, stop combination. A personal phobia, maybe. But also thanks to new technology. Instead of heaving stops in and out while the audience thinks you're pausing for musical expression, stop changes are now computerised. You just set your combinations on a memory card (999 combinations possible, though an average recital needs only about 150) and hey presto]
Stop names give little idea of the sound of an organ; and it's not just how they sound individually, but how they sound in chorus. Simon footles with the 32' Untersatz . . . Waah, waah, brrh . . . What a foul noise, like a dinosaur farting. 'But with full organ it makes the sound solid.' BLAAH BLAAH, DA DAAGH] Agreed.
Isn't there a danger that modern concert organs are becoming homogenised, all trying to find the best way of coping with the widest range of music? Simon dismisses this: even on a smallish organ like St John's, there's a wide variety of solo and quiet stops. 'Here are some French stops . . . Pure incense, don't you think? And a very good range of flutes . . . a quiet open Rohrflute, a Doppelflute . . . two lips, very rich, warm, helpful in building sound . . . just like an English organ] This organ will sound very different playing Bach from playing Bossi, Bonnet, Tournemire, Franck and Messiaen. It's only when you use full organ that it will sound the same, because everything has to pull its weight.'
Can he tune the organ himself? 'I just wave my hand over the pipes, until I find the one that goes EEOOOW . . .' And voicing? 'That's where I get VERY interested. Of course I was here when Christoph set the level for the first rank - which dictates how big the organ will sound.'
Now the last, worst question. If other people think this organ is too loud, can it be revoiced? 'Yeeeesss,' he concedes, as though the Dean has suggested an anthem might be out of keeping. 'Christoph could soften the pipes by closing the mouth down, or closing the foot so less air goes through. But he'd have to revoice the whole organ, which he'd be loth to do, having spent so long getting it EXACTLY right]'
So don't argue with the experts. Enjoy the mystery. Each organ is different, each makes its own rules, according to an exact combination of architecture, acoustics, builder, player, prefabricated but flexible, free-standing pipes, and the ears and expectations of the audience. All that before you play any music.
The Sainsbury organ in St John's Smith Square will be inaugurated at a gala concert on Tuesday. On Wednesday at 1pm, Simon Preston - playing a Bach programme - begins a series of midday and evening organ recitals featuring Gillian Weir (14/15 October), Daniel Chorzempa (28/29 October), Jon Laukvik (4/5 November) and Alexander Fiseisky (23/24 November)