Outside Edge: Sabine Durrant met piano-tuning's Caped Crusader, Peter Salisbury

PIANO-TUNERS used to ride bicycles. Usually doddery, often short-sighted, they'd stop for tea and mutter darkly about the perils of central heating. Things are different now. Peter Salisbury, 34, drinks coffee - black. He wears leathers and tight T-shirts and he carries a packed toolbox. In it are filing sticks, hot irons and solvents, capstan turners and lead weights. If he's in his car, there'll be a piano jack in the boot. Usually, though, for the purposes of speed, he's on his bike: a BMW 800cc - 'same as the police,' he says.

He's a sort of one-man emergency service. Last year, when a pianist had a problem with his pedals at a concert in Sion Park, Salisbury was there within the hour. 'I burst on to the stage, I cleared the area, I repaired the damper, then I disappeared off again, zooming out on my motorbike,' he recalls. 'The promoter told me afterwards I looked like a cross between the paramedics and that RAC advert. He calls me the Caped Crusader.'

At the South Bank and the Wigmore Hall, they have many other names for Peter Salisbury. The official title is Concert Piano Technician (Freelance). But one of the administrators at the Festival Hall describes him as 'more of a marriage broker, really. He introduces visiting pianists to our six pianos and helps them choose. A sort of match-maker.' But Salisbury prefers to think of himself as an English butler, 'who keeps quiet, does discreet things . . .' Pianists tend to keep their sheets to their chests, he says. 'They like me to keep what I do to myself. If you want details, you'll have to wheedle them out of me.'

Wheedling then, Salisbury - who had an apprenticeship in the service division of Steinway at the age of 16 - doesn't just tune, he works pianos. He's half-mechanic, half-musician. 'You've got to understand the piano backwards and forwards, inside and out,' he says, rubbing his gingery beard, 'but you've also got to understand what sound the pianist is trying to achieve. I can tell within minutes - by which part of the body they're using to attain the tone - what they're going to ask for. Most pianists, unlike flautists or violinists, have no idea what goes on in their instrument - 'It's clunking,' they say, or 'It's whirring' - but they know what they want.'

Minutes of observation - followed by hours of 'discreet' consultation - have taught him that Alfred Brendel likes 'a bright clear voice with a traditional Steinway regulation, but on the shallower side'; that Shura Cherkassky prefers 'a rounder sound, more malleable, with a wider tendency for colour'; that Alicia de Larrocha prefers her tuning to be 'very slightly distorted, to give the piano a warmth, to give the Steinway - she's definitely a Steinway pianist - a Bechstein sound'. That's all he'll say, though. The rest comes under his 'ethics of confidence'.

Sometimes, Salisbury can spend months at the South Bank in the service of one visiting pianist, tinkering with the levers and the keys, altering the tolerance of the springs or the felt. When they've left, though, he always puts it back to how it was before. 'You have to: that pianist will say it's the best piano they've ever seen, but the next one will come along and say, 'But this is ghastly]' They have no perspective, pianists, no idea that a colleague will see an instrument differently. And that's why they need me. I stand on the sidelines, providing objectivity.'

Before he gets on his BMW and revs up for the Henry Wood Hall, Salisbury has one last shot at self-definition. 'I suppose I'm a sort of linesman,' he says.