A Virgin Atlantic Airbus A340-600, registration G-VGAS performing flight VS-9 from London Heathrow,EN (UK) to New York JFK,NY (USA), was enroute at FL400 about 170nm southwest of Halifax,NS (Canada) when problems with the cabin pressure prompted the crew to descend the aircraft to FL240. The aircraft continued to New York for a safe landing about 65 minutes later. The AAIB reported that a number of crew were temporarily affected when cabin pressure problems prompted a controlled descent. The occurrence was rated a serious incident and is being investigated by the AAIU.