There was a time when nostalgia was a sketchy word, shorthand for the sentimentality whose flipside was memorably nailed by legendary art critic Robert Hughes as viciousness, vivid in the racism, sexism and homophobia which often underpin a yearning for “the good old days.” But nostalgia had a reboot during London Fashion Week’s 40th anniversary, starting with the spectacular retrospective of the NEWGEN fashion incubator which Sarah…