TWENTY-TWO YEARS AGO, R&B SINGER ERROL BROWN RECALLS, he was strumming a funky little melody on a guitar in his London bachelor's fiat when the words magically popped into his head and he began singing," I believe in miracles/ Where you from?/ You sexy thing...." In 1976 his band Hot Chocolate, which John Lennon had signed to the Beatles' Apple Records label five years earlier, turned "You Sexy Thing" into a hit that reached No. 3 in the U.S. and No. 2 in the U.K. before disappearing, along with Afros, disco and polyester suits, into the pop-culture attic. But thanks to featured play on the film sound tracks of The Full Monty and Boogie Nights, plus a new Burger King commercial, the song, like many '70s phenomena, is enjoying a resurgence. Now, with "You Sexy Thing" back on U.K. pop charts and about to be rereleased here, Brown, 47, is pleased that his ditty has found new life. "It's the first happy one I wrote," says Brown, who disbanded the group in 1985 and retired two years later to a quiet life in Surrey with Ginette, his wife of 20 years, and their two daughters. "Following so many years of pressure, I've been quite happy to chill out with my family," he says. But Brown, who capitalized on Monty-mania with a six-week autumn tour, is considering a trip to the States. "I'm amazed young people love the song," he says. "It's like showing off one of your children."