When Steve Winwood was 16 years old he was a rock star in England, as vocalist, pianist and guitarist with the Spencer Davis Group. When he was 20 he made an impressive American debut with Traffic, one of the finest and most influential progressive rock bands of the 1960's. Traffic rapidly won a large and loyal American following but by 1975, when Mr. Winwood was only 26, he had lived through 12 years on the road and felt he badly needed a rest. So Traffic disbanded, he bought a small farm north of London, and little was heard from him until last week, when his new solo album, ''Arc of a Diver,'' was released by Island Records.

Steve Winwood climbs onto a small balcony and peers through the curtain at the keyboard of London's Royal Albert Hall organ. The only time he's ever seen this beast played, he says, was when the Mothers of Invention did "Louie Louie" on it. When was that now, '66? '67? Nobody quite remembers.

"And when did you last play here, Steve?" asks someone else. He looks upward and squints at the flying saucers in the Albert's roof, as if trying to find a date to fit the image. But no. He doesn't remember that either. He laughs, amused at his own vagueness.