Music: Nicholas Roe persuaded the London Philharmonic to let him join their percussion section for one concert. As rehearsals progressed, he realised he was 78 bars away from triumph ... or disaster
Rachel Gledhill said "follow me" and led the way down backstage corridors in London's Royal Festival Hall to a broom cupboard. Here I had my audition for the London Philharmonic Orchestra.

Technically it was an equipment room, but that's just semantics. It was a tiny space crammed with boxes, except for four square feet near the door where we squeezed in and Rachel, principal percussionist, set up a bright silver triangle on a stand.

"Play this," she ordered, handing over some music. But by now my hands were shaking and my shirt was drenched unpersonably in sweat; she might as well have asked me to play the tuba with my nose. But that was the point.

I had wanted to peer behind the cloak that separates concert-goers from a human understanding of orchestral performance. But I've only got a measly grade four piano. After much debate, the LPO agreed to let me play the triangle in one concert if I passed the audition. So here we were.

Rachel watched me go digga-digga on the bright steel in semi-quavers. You have to be facing the complexity of orchestral sheet music and experience the tension created by an audience of one to understand how hard that was. Rachel said, "I should use one hand instead of two. And try holding the rubber bit" - which sounded funny, so I almost laughed. But finally she said, "You'll do" and the dream was on.

In three days' time I was to play at the Festival Hall in one of the LPO's hour-long family concerts. My part was not huge, but scary. It was from Rimsky-Korsakov's Capriccio espagnole: play 13 bars, rest for 12, play 14, rest 13, then play 12 very softly indeed. And repeat. Seventy- eight bars in total, or 624 "diggas", which I practised with the triangle suspended from a saucepan on the mantelpiece weighted down with a copy of The Writers and Artists Yearbook.

Two days later I attended the first rehearsal. In a vast, chilly hall in Blackheath, south London, musicians wandered in from a grey Saturday morning chattering about parking spaces and laughing, They were ... casual. Yes, that's the word. Casual in their woolly jumpers, their jokes, their instant friendliness - even their skill.

Imagine the lay-out of an orchestra, with percussion at the outermost rim like stroppy kids at the back of the class. Each member of this section must master scores of instruments, often swapping places. By osmosis it seemed (I could never read the conductor's winks and nods) the orchestra suddenly understood that they were to play Falla's Ritual Fire Dance, the opening bars of which feature a xylophone crashing over the top. Except this was first thing in the morning and the percussionist due to play the xylophone, Andy Barclay, had not arrived.

The orchestra rustled, the jovial conductor Bramwell Tovey, miles away down there in front, began to raise his baton. At this point, Keith Millar, another percussionist, who had been offering me some witty advice on pre- concert nerves, suddenly murmured "Excuse me", strode three paces to the gleaming xylophone and snatched up the sticks. The baton flicked down; Millar began playing fast, furious, perfect music.

Nobody made anything of it. Two minutes later, Andy Barclay ran in, cursing a failed railway system, scooped up the sticks, and played the second run-through with such bravura that the harpist, Rachel Masters, whispered, ironically, "Very nice".

When you are within the gut of an orchestra, even in rehearsal, it is hard not to be overwhelmed by the sheer sound. It's uplifting, but disproportioned, your own section looming hugely a milli-second before the rest. It's a weird feeling, wonderful but a bit savage, as if the final refinement takes place somewhere in the air between you and where the audience will be. And with all this going on, you must say to yourself: one, two, three, four ... digga-digga.

The second violinist, Fiona Higham, puts this into still more human context. Fiona has two young children. She had played a concert the night before, was rehearsing all Saturday morning, practising alone all Saturday afternoon, performing at a concert in Eastbourne on Saturday night, rehearsing on Sunday morning - and then playing in the family concert.

During rehearsals Rachel said, "Look, have a go at the Kabalevsky too ..." thus adding another 19 breathless bars to my repertoire. The trouble with this second piece was that it required bashing the triangle for 16 bars then counting a crucial rest before a loud finale. One missed beat and you had an unplanned solo on your hands.

Fear? Yep. The others admitted only to a certain tension, worse in a grand performance - Mahler, for instance. But at family concert level, they were merely keyed up. As Andy Barclay put it, "I very rarely get nerves about things that are difficult because I have a shit or bust mentality about that. But I do get nerves about exposed, very quiet things. And everyone has bad days." But do they suffer absolute, steaming, literal funk? I think not.

Now it was performance time. Backstage was alive with fierce chatter; musicians filling every nook practising lonely parts. Minutes ticked away. Rachel mentioned that her two-year-old son, Luke, had kept her awake most of last night (this, after concerts on two consecutive nights); then she laughed wildly and was gone.

Someone said I should also go in now, and when I walked up on to the platform where the rest of the percussion section were set to watch over me like hawks it was like drowning: a tangible pressure of 1,500 people and eyes.

The orchestra had subtly altered. Nobody was casual any more. The players were like a pack of dogs before a hunt: sprawncy, yet controlled. A hush, a raised baton ...

I hit the beat okay; didn't get stuck with an awkward solo. I felt incredibly alive, part of an organism and yet an individual, just. I imagine that this is what they all love: the semi-protected glory.

In the middle of Capriccio espagnole a set of castanets broke and guest percussionist Alex Neal cooly switched to another without missing a beat: ole! Also, when I finished my 78 bars Andy Barclay whipped the page over ... then paused long enough to give me a thumbs-up under cover of the stand.

But what really mattered was watching Rachel Gledhill take a personal bow for a particularly difficult piece ... on triangle. Musicians don't paddle in applause; they bathe in it. That's where they score over mortals who must make do with a word from the boss. Rachel looked entranced. So was I, of course. Though the dream was ending now, and four hours later I felt terribly depressedn