So just to make life more complicated my original flight has been cancelled. The GGL line rebooked me on to an earlier flight and reissued my ticket. But same problem occured - could not check-in or access MMB. So called again and got an agent who knew how to unlock the booking. He still couldn't tell me why it was being locked (someone or something was trying to access my booking with incorrect details and it locks out). But he knew how to unlock it. He said locked bookings can be released immediately, unlike a locked EC account which requires a 24 hour delay apparently.