By WALT TROTT Staff WriterTHE TABLES were nearly turned in Frankfurt last Saturday night as a warm-up English group, Gentle Giant, proved nearly as popular with fans as the main attraction, Jethro Tull-Thousand of concert-goers demanded encores from the lesser-known sextet after more than an hour of their pleasing sounds and dynamic showmanship — before letting them make way for the top-billed Tull.HAILED BY THE rock music world as one of the more exciting international groups, the British Jethro Tull gave their faithful fans a generous heaping of their usual, fast-paced, musical expertise.The talented Tull consists of leader Ian Anderson, a flutist, vocalist and musical arranger; Clive Bunker, drummer; Martin Lancelot Barre, lead guitarist; Jeffrey Hammond-Hammond, bass and John Evan, pianist. (Two former musicians, Mick Abrahams and Glenn Cornick, have joined other groups, Anderson said.)Anderson, who guided the group to its present world-wide popularity, is a wide-eyed, wild-haired, somewhat flam-buoyant Scotsman, who calls Tull’s music “progressively pop.”HE SAYS THE name, Jethro Tull, was borrowed from an 18th century British agriculturist who invented the seed drill, but that it has no real significance.Tull rose to prominence about fourTull Gets Giant Warm-upyears ago with a smash appearance at the National Jazz and Blues Festival Stateside. The group followed this with the hit LP, “This Was,” and successful record singles, “Witch’s Promise” and “Sweet Dreams” that quickly caught the fancy of rock music fans.Prior to his Frankfurt show, Anderson disclosed that his attractive, darkhaired wife, Jennie, penned the title tune from their latest hit album, “Aqualung.”THE SCOTSMAN pointed out that Mrs. Anderson tried to relate, through a single character, to the world’s castoffs — the pariah that the historical God made room for; but, who remained rejected by other people. (The name of the derelict in question is inspired by his respiratory trouble.)His female counterpart is “Cross-Eyed Mary,” a kind of latter-day, school-yard Magdalene. “Aqualung” has conveniently been described as a pro-God, anti-Church statement.Of his own God, Anderson says, “He’s not the kind you have to wind up on Sundays.”This musical genius has been variously praised in both Life and Playboy magazines. The latter’s music poll rated Anderson as the world’s top flutist.PERSONAL APPEARANCES enhance the popularity of Jethro Tull, with the carrot-topped leader being good-naturedly labeled “a mad-dog Fagin” and “a demented dancing master,” by the news media.When asked if the group had experienced the usual difficulties on the road to success, Anderson retorted, “No, we had good management. We didn’t mess around and knew what to do.”About the future: “Everyday brings its own satisfaction and challenges. We want to retain our involvement with music, the level we have had to date, but not let boredom set in.“Therefore, we have to avoid what we did last year ... be musically different this year,” concluded Anderson.However, this is not the problem for Gentle Giant, whose initial appearances in Europe have shown great promise. Although the concert managers cut short their encores to bring on the star group, Gentle Giant obviously was a hit with the fans.Gentle Giant, a promising British group.