Britain's greatest showman of today is paying lavish tribute to America's greatest showman of yesterday, and by all accounts the upcoming musical "Ziegfeld" will merit one of Harold Fielding's favorite words spectacular.

"I've always believed that if the public is going to give up its television sets for an evening at the theater, then you should give them as much on the stage as you can," Fielding said the other day.This belief has made Fielding, at 71, the uncrowned king of musicals in Britain. By comparison Andrew Lloyd Webber is a promising newcomer. "Ziegfeld" is Fielding's 71st London show, the capstone of 30 years of musicals and his most ambitious yet.

"It's one of my extravaganzas," Fielding said.

In Britain that means something. Fielding is famous for bringing the biggest of big names to British audiences from Paul Robeson and Jeanette MacDonald to Judy Garland and Frank Sinatra and for re-creating Broadway musicals on a lavish, hang-the-expense scale.

This time he's importing Tony-winning Broadway star Len Cariou (named best actor in 1979 for "Sweeney Todd") to portray the legendary Florenz Ziegfeld, whose "Follies" dazzled Broadway between 1907 and 1931 and became an American institution.

The musical, opening April 26, "tells quite a lot about Ziegfeld and his life," Fielding said. It also tries to reproduce the opulence of Ziegfeld's shows.

So it will have showgirls rising aloft on 16-foot columns, 27 scenes, some 400 gowns which alone cost $1.85 million.'

"It will be the most spectacular escapist show I've ever produced," Fielding said.

He fairly bursts with enthusiasm while saying it, facing the grand piano in an ornate office dating from 1723. But then, Fielding always has been irrepressible.

He's a gnome of a man, barely 5 feet tall. In Snow White's lineup of seven dwarfs, Fielding would be Happy. He races through 21-hour days "I can get by on two or three hours' sleep," he said and tackles each show with a schoolboy's verve.

He rarely seeks the limelight. "I want to push my shows, not myself," he said. It was different in his youth.

Then Fielding was a violin prodigy, giving concerts at the age of 12. In his early 20s he began promoting concerts as well as playing in them. Before long he focused on London stage shows with even bigger names.

"All the great stars of the world have passed through my hands at one time or another," he says.

His first real musical, "Cinderella," came in 1958. Ever since, through shows like "Barnum" and "Half a Sixpence" and "Singing in the Rain," Fielding has been responsible for the tiniest detail of every production, down to the last sequin.

"You've got to be at every audition, in and out of the box office, at all the workshops, or you lose the pulse," he said, black lizard shoes flashing as he crossed his legs.

His collaborator on "Ziegfeld" is American choreographer and director Joe Layton, another Tony winner. His cast includes Australian dancer Amanda Rickard and French singer Fabienne Guyon. But the decision maker behind the show's 250-person team is Harold Fielding.

"You think about a show for a long time, but at a certain moment you have to press the button," he said. "And from that moment the adrenalin flows."

Unusually, Fielding invests his own money in his shows.

"I have never, never disclosed how I do these things, but I'm the greatest gambler in the world," he said. "In the theater, not on the race track.

"I just don't think I could ask anyone I know to put money into my shows. Shows are a terrible gamble. I've had some tremendous flops."

More on his mind that day, however, was his triumph in wringing from 99-year-old Irving Berlin permission to use what was practically Ziegfeld's theme song, "A Pretty Girl is Like a Melody."

"He will only allow us to use this one once, it can't be part of the overture, we can't do a reprise," Fielding said. "But we've got it."

Fielding confesses easily that "I'm not a highbrow: I don't understand the classics." All his musicals are pitched down the unoffensive middle, at "the family audience."

This can mean unabashed corn. "Ziegfeld" will end with Cariou's showman ascending to heaven on a Hollywood-style staircase.

But the formula can be a box office winner. "Ziegfeld" has set an all-time London Palladium advance sale record and Fielding is entirely unapologetic.

"In the end," said the king of British musicals, "it's just another show. Whether it lives or dies is in the lap of the gods.

"I wouldn't attempt to compare myself to Ziegfeld. But we both liked the same things: great beauty on the stage, glamor, spectacle and girls.

"Whatever happens, if they put on my tombstone `He was a good showman,' that would satisfy me."