The sun came out in London two weeks ago and so did the mini.

The 1981 mini is not the same mini as in the swinging '60s, when Courreges outdated the long skirt with clean, architectural styles in Le Corbusier colors, or when Mary Quant's minis, once pegged as "the gym slip of the permissive society," clothed that decade's youth-oriented society.

Now the mini separates the young from the old, with a few older women with mighty good legs opting to wear it. And it is an optional item to suit one's mood, to be worn alternately with pants, which can be any length or shape, or a long folkloric skirt.

And unlike before, the mini is only part of the high fashion wardrobe. It is being worn in much more casual ways, always with tights and flats or flat boots, never with spike heels. It often looks more like a tunic with tights than a very short skirt.

Many models and some every fashion-conscious young women have begun racing about New York and London in short skirts. The knee-length pants or culottes, particularly ones designed by Giorgio Armani, have become the pet of fashion loyalists who are quick to pick up fresh looks.

"With the young, the mini is almost a norm," suggests Catherine di Montezemolo, Lord & Taylor vice president. "It won't be as important for fall as in warm weather," she says. "But for the moment the miniskirt worn with romantic blouses with full sleeves and soft crushed boots make one look like a Sherwood Forest creature. It's charming."

The mini revival, which should be full-blown this summer, stirred more than a year ago when the big, sweatery mini-dresses were picked up by kids who wore them with thick, colorful pantyhose. An older crowd wore some of the same tops, mostly by Kenzo and Dorothee Bis, over narrow pants or shorts.

In London the black plastic mini has been part of the punk uniform of youth for the past four years. And now, as punk style and attitude is diminishing, the miniskirt is reappearing on the young who find it a welcome weekend alternative.

There are those in the United States who have never given up the miniskirt, but that has nothing to do with fashion. It was virtually wiped out in by the early 1970s by the hippie gypsy styles -- pants and jeans particularly -- and the few attempts to bring it back have never amounted to much. In an era where success in the board room counted more than turning heads on the street, the mini hasn't stood much chance.

"The revived miniskirt is not defiantly short," observes Bernie Ozer, of Associated Merchandising Corp. "It is strictly an alternative, strictly for the younger woman or a rare older woman who wants to show off her good legs. There is no suggestion that everyone has to wear one."

Perry Ellis, who has shown both very long and very short skirts for some time now, is showing his mini length as shorts or culottes, not skirts, for fall. "I've liked them [short lengths] for a long time and I think they are right for one day, while longer skirts may feel right the next. But length is not an issue. Both the long and the short really look beautiful."