This invention presents a locking system (112) for a closure. The system comprises a casement lock (112) having a primary lock (116) moveable between secured and retracted position, a primary actuator (122) for movement of the primary lock, a secondary lock operable to prevent movement of the primary lock, and an opening (124) for receiving a cylinder lock (102) for selectively actuating the secondary lock. The cylinder lock (102) comprises a top section having a barrel with a through passage at either end, the barrels being separated by a cam opening. A lock (108) is provided at each end in the through passages, and a cam member (106) is located between the locks. A lower section extends along the length of the cylinder lock and has a bridge section joining the barrels at either end across the cam opening. The width of the top section is equal to the width of the lower section. The system also has an adaptor block (114) receivable in the opening to change its shape such that, with the adaptor block (114) in situ, the shape of the opening conforms to a euro profile cylinder lock. (Figure 4)