A locking system for a closure, the system comprising: a casement lock comprising: a primary locking means moveable between secured and retracted positions in respective first and second directions; primary actuation means for selective movement of the primary locking means in the first and second directions; secondary locking means selectively operable to prevent movement of the primary locking means in the second direction; the casement lock further comprising an opening therein for receiving a cylinder lock therethrough for selectively actuating the secondary locking means; and a cylinder lock (2, 102) comprising: a first section comprising a barrel having a through passage at either end, said barrel separated by a cam opening; a lock provided at each end in the through passages, and a cam member located between the locks; and a bridging section extending along the length of the cylinder lock and joining the barrels across the cam opening; and wherein the width of the opening adjacent the first section is substantially equal to the width of the opening adjacent the bridging section; and wherein the locking system is further provided with at least a first adaptor block receivable and removable in said opening to modify the opening size such that with the adaptor block in situ the shape of the opening substantially conforms to the shape of a euro profile cylinder lock; and characterised in that said first adaptor block modifies the opening size to reduce its height to a first height. a casement lock comprising: a primary locking means moveable between secured and retracted positions in respective first and second directions; primary actuation means for selective movement of the primary locking means in the first and second directions; secondary locking means selectively operable to prevent movement of the primary locking means in the second direction; the casement lock further comprising an opening therein for receiving a cylinder lock therethrough for selectively actuating the secondary locking means; and a primary locking means moveable between secured and retracted positions in respective first and second directions; primary actuation means for selective movement of the primary locking means in the first and second directions; secondary locking means selectively operable to prevent movement of the primary locking means in the second direction; the casement lock further comprising an opening therein for receiving a cylinder lock therethrough for selectively actuating the secondary locking means; and a cylinder lock (2, 102) comprising: a first section comprising a barrel having a through passage at either end, said barrel separated by a cam opening; a lock provided at each end in the through passages, and a cam member located between the locks; and a bridging section extending along the length of the cylinder lock and joining the barrels across the cam opening; and a first section comprising a barrel having a through passage at either end, said barrel separated by a cam opening; a lock provided at each end in the through passages, and a cam member located between the locks; and a bridging section extending along the length of the cylinder lock and joining the barrels across the cam opening; and wherein the width of the opening adjacent the first section is substantially equal to the width of the opening adjacent the bridging section; and wherein the locking system is further provided with at least a first adaptor block receivable and removable in said opening to modify the opening size such that with the adaptor block in situ the shape of the opening substantially conforms to the shape of a euro profile cylinder lock; and characterised in that said first adaptor block modifies the opening size to reduce its height to a first height.