Stena Line, the leading ferry operator, has announced it will be offering 15 per cent off bookings made between 14 and 28 May 2011, for travel completed by 8 January 2012 to mark the Passenger Shipping Association's third annual National Ferry Fortnight. The offer is valid for all vehicle types, "dutchflyer" (rail and sail) and foot passenger bookings, as well as all elements of booking from cabins to pre-booked meals and Stena Plus lounge access. The 15% discount on travel to Holland offer means a family of four (two adults…