Passengers on a London-bound Cathay Pacific flight spent 4½ hours in the air before landing back in Hong Kong after the aircraft experienced problems with its water supply.Flight CX253 departed from Hong Kong International Airport for the 13-hour trip to Heathrow at 2.40pm on Monday. But a problem with the aircraft’s potable water supply meant the flight returned to Hong Kong as a standard precautionary measure, a Cathay spokesman said.The Boeing 777-300ER, which has a capacity of 398…