The Portuguese have a word for it: saudade. In his essay, The Secret Life of the Love Song, singer Nick Cave defines it as 'an inexplicable longing, an unnamed and enigmatic yearning of the soul'. If you want to hear a peculiarly English version of saudade, you need only listen to Robert Wyatt singing 'At Last I Am Free', a soul ballad initially made famous by Chic, arguably the greatest disco group of the Seventies. In Wyatt's hands, an already stoical and lovelorn lyric - 'At last I am free, I can hardly see in front of me' - is imbued with an almost existential sense of aloneness. It positively drips saudade.
Slightly broken, inexpressibly plaintive, Wyatt's voice was once described by the actor and musician, Ryuichi Sakamoto, as 'the saddest sound in the world'. In the Seventies, it transformed Neil Diamond's 'I'm A Believer', originally a hit for The Monkees, into a hymn to romantic bemusement, and, most famously, made Elvis Costello's 'Shipbuilding', written as warships sailed to the Falklands, the most powerful anti-war song of recent times. It is, almost by accident, an instrument of great and melancholic beauty; a musical signature that is as recognisable, and as heart stopping, as Miles Davis's muted trumpet or Billie Holiday's bruised, world-weary voice.
'I've never tried to express myself in my life,' Wyatt retorts, when I clumsily attempt to convince him he has a definite and unmistakable style, 'even where I seem to be doing just that. If I use something from my past experience, it's because I think it would mean something to me as a stranger. I never just empty my heart on to a tape. It's difficult to put into words but I believe people have their own feelings that I don't want to tamper with, or manipulate in any way. All I know is that I try to do this, that and the other, and it just comes out the way it does. What I do is severely limited - that, if anything, is the style.'
Next month, Wyatt receives a long overdue canonisation of sorts as curator of this year's Meltdown Festival. 'It's still sort of unbelievable,' he mutters, looking genuinely gobsmacked by the enormity of the undertaking. 'I mean, I live in my own sort of cut-off world. I don't gig or record much or collaborate much anymore and then this enormous thing lands on my lap. The words "money" and "old rope" keep leaping into my head.'
Wyatt's Meltdown is, even by previous standards, an ambitious and challenging event. It reflects his abiding interest in jazz of the experimental variety, past and present cutting-edge music from across the globe, and left-wing politics.
'You could make a day of it down by the river,' Wyatt muses, as if talking about a village fête, which is what Meltdown is, albeit on a global level. 'Maybe grab a bite, listen to some music, have a laugh or two. There is,' he adds, his still slightly discernible West Country tones giving way to a strange semi-cockney, 'sommit for everyone. Really and truly.' His only regret is that he could not contact the all-girl pop group, Eternal, whom he loves, and one imagines would have, in an ideal world, paired with Naomi Klein, his latest inspiration.
Wyatt, then, is not your typical pop singer. He is a card-carrying Communist - 'I read Chomsky and that lot, and I just thought, these are my people, this is exactly how I see the world' - and has relocated to deepest rural Lincolnshire after a spell in Spain. Now 56, he has been wheelchair-bound since 1973, when, drunk, he fell from a window to the street below. Until then, he was a drummer who occasionally sang, having played in Soft Machine, the archetypal serious prog-rock group of the late Sixties, then forming the even more left-field Matching Mole. His first solo album was End of An Ear, and, according to the Virgin Encyclopedia of Rock Music, 'established a style that merged the avant-garde with English eccentricity', which is, more or less, where he has stayed ever since, waiting for the world to catch up.
'I don't really write proper, crafted songs,' he confides, 'not Like Randy Newman or Elvis Costello. Mine are strange little things, aren't they? They always seem to come out funny.'
I first interviewed Wyatt in December 1985 when, as he keeps telling me, I was 'a little cub reporter sort of chap for NME'. It was, in fact, my first cover story, and I still have it, framed and pristine, on the wall. Wyatt's second solo album, Rock Bottom, released in 1974, is one of my favourite records. Composed while he lay immobile in hospital after his accident, it still sounds like nothing I have heard before or since: a tonal reflection of his submerged, dislocated state of mind, part dreamy, floating soundscape, part affirmation of an unbroken will to create. Its oceanic tug and sway is mesmeric, conjuring up an impressionistic landscape of nuance and suggestion way beyond the narrow confines of the archetypal rock song.
'Well, I was literally cut off when I made it,' he reflects. 'I was away from the traffic of being in a pop group so I just floated off on my own. It was like that moment when you hit the deck and go to sleep, you don't suddenly start dreaming, but real life starts to unravel. I think I was like that in a waking state back then. I just turned on the taps and let if flow.'
Born in 1945 in Bristol, Wyatt grew up listening to his father's record collection - classical composers such as Ravel and Hindemith shared space with Duke Ellington. He graduated to Charlie Parker and John Coltrane, but insists that hearing the strident funk of early James Brown and the Stax label changed his life. 'It was an epiphany. Before that, it never occurred to me that a mere mortal like myself could play music. It just freed everything up.'
After Soft Machine's excursions into cerebral jazz-rock, he seems to have wilfully recast himself as a pop performer, albeit an effortlessly left-field one. 'I was always a bit suspicious of rock. My theory was that pop was one of those words like perfume that men were a bit shy of, so they had to invent aftershave to sell it to them. They had to invent rock as men's version of pop.'
Since his accident, his solo albums have been fitful, his long periods of silence often punctuated by unlikely collaborations and cover versions: the stirring Eighties jazz-folk song, 'Venceremos', alongside Working Week and singer Tracey Thorn; a solo reading of 'The Red Flag'; a techno-folk tune with south London duo, Ultramarine. In 1997, he released Shleep, another beautiful and strange album produced by fellow pop thinker, Brian Eno. On it, he sings a song called 'The Duchess', the latest ode to his wife and occasional co-writer, Alfreda Benge, whose understated paintings grace his album covers. Live performance, though, remains a no-go area: 'Can't do it,' he says, matter of factly. 'No. No. Definitely not. The nerves. I used to have to get drunk to go on stage.'
I ask him if he was a big drinker back in the old days. 'Yea. I suppose so. That's how I fell out the bloody window. It's that thing that Sonny Rollins said about performers - where we work is where other people play. It's an environment where drink, and everything else, flows freely. It was just available. I had the opportunity to drink, and I did. A lot.'
Recently, someone sent him a CD of a Matching Mole gig in Paris, and he was surprised at 'how much fun I was having behind the kit'. After the fall, though, he stopped drumming altogether and began composing on the keyboard. He thumps the piano as you imagine a percussionist would, bashing out what sounds like a typical Robert Wyatt chord sequence, if typical were a word you could apply to someone this singular. Then he recites a song he is working on, called 'Unamerican Activities', where every line is punctuated with emphatically English words like footpath, sweets and dustbins instead of pavement, candy and trash cans. 'It's silly but it has a point. I tend to work in my own head like that. I see myself as just being the first person who happens to hear my music. I'm like an editor, really, filtering out the crap, clinging on to the bits that might work, that I might want to hear again. I really don't think of it as creating in any meaningful sense of the word.'
And yet, this humble, eccentric, utterly unique musical maverick has created some of the most timeless and singular work in pop's long and varied history, songs that require you to maybe listen at a different angle, but will make you see the world in a different light. 'My inspiration is anything that will help get rid of the cold chill of loneliness of being on the wrong planet,' he muses at one point, which is as good a description of his own inspiring soundscapes as any I can think of. Long may he fiddle about.
Meltdown is at the Royal Festival Hall, London SE1, 7-27 June. Soup Songs, a tribute to Robert Wyatt, featuring Elvis Costello, Brett Anderson, Julie Tippett and others is on 24 June. Call 020 7960 4242 or www.rfh.org.uk/meltdown