BACK in the news is our old friend Colonel John Blashford-Snell, the intrepid British explorer who is ofttimes likened to a latter day Indiana Jones. Blashford-Snell, or ''Blashers'' as he is known to those of us who have had occasion to share a tent with him in the Gobi Desert, has just returned from his latest expedition to Mongolia. And in his inimitable style he has been dining out on the story of how he found himself chasing giant rats - although what his animated narration did to the appetites of his fellow dinner guests is another matter. But the most spine-chilling moment of his latest venture out East came, he told Keeping Posted, during a stopover at Kai Tak Airport when he was met at the arrivals lounge by a man waving a piece of cardboard reading ''Colonial Schnell''. The fact that Blashers lived to tell the tale is testimony to his fearless courage.