CHRISTMAS has officially been declared at the Keeping Posted cubicle with the arrival of a gift from John Duffus, who handles matters for the prolific (in making both music and money) Andrew Lloyd Webber in our neck of the wood. While keeping busy-busy with the fast nearing January opening of Cats in Hong Kong, Duffus - MD of The Really Useful Company in Asia - was in Japan a few weeks back for the opening of The Phantom of the Opera in Sapporo when he thought the ubiquitous official T-shirt of the show would make the perfect gift. The garment is very presentable indeed. And, like all theatrical merchandise, even contains a couple of lines from the show. But someone should have been on hand to tell the designer that while some lyrics from the show make perfect sense on stage, in isolation on a white T-shirt the words have a habit of taking on a whole new meaning. So it reads: ''She chased the Phantom around the opera house, until she caught him by the organ. . .'' Ouch!