A method of actuating at least one of a plurality of downhole tools (10A-J, 50) connected to at least one hydraulic fluid line (L1, L2), wherein the method comprises the steps of:- a) providing each of the downhole tools (10A-J, 50) with a control unit (20A-J, 55A-J, 56, 58A-B) comprising a timer (55A-J, 56, 58A-B) and a valve (30A-J) operable by the control unit (20A-J, 55A-J, 56, 58A-B), characterised in that the control unit (20A-20J, 55A-J, 56, 58A-B) is capable of opening the valve (30A-J) at the start of a window of time (WO1, WO2) determined by the timer (55A-J, 56, 58A-B), wherein the timer is arranged such that said timer times the window of time subsequent to a known instance in time (T=0), such that:- i) the control unit (20A-J, 55A-J, 56, 58A-B) is capable of permitting fluid communication of hydraulic fluid between the at least one hydraulic fluid line (L1, L2) through the open valve (30A-J) such that it is communicable to the downhole tool (10A-J) during the said window of time (WO1, WO2); and ii) wherein the control unit (20A-J, 55A-J, 56, 58A-B) is further capable of closing the valve (30A-30J) at the end of the window of time (WO1, WO2) determined by the timer (55A-J, 56, 58A-B) such that the control unit (20A-J, 55A-J, 56, 58A-B) prevents hydraulic fluid from flowing through the closed valve (30A-J) and the hydraulic fluid is thereby prevented from acting upon the respective downhole tool (10A-J, 50A-J) at all times outwith the said window of time (WO1, WO2); and b) controlling the pressure in the at least one hydraulic fluid line (L1, L2) for at least a sufficient period of time required to at least actuate the said downhole tool (10A-10J, 50A-J), wherein the said sufficient period of time coincides at least partially with the said window of time (WO1, WO2) such that the downhole tool (10A-J, 50A-J) is capable of being actuated during the said window of time (WO1, WO2) and is prevented from being actuated at all times outwith the said window of time (WO1, WO2). a) providing each of the downhole tools (10A-J, 50) with a control unit (20A-J, 55A-J, 56, 58A-B) comprising a timer (55A-J, 56, 58A-B) and a valve (30A-J) operable by the control unit (20A-J, 55A-J, 56, 58A-B), characterised in that the control unit (20A-20J, 55A-J, 56, 58A-B) is capable of opening the valve (30A-J) at the start of a window of time (WO1, WO2) determined by the timer (55A-J, 56, 58A-B), wherein the timer is arranged such that said timer times the window of time subsequent to a known instance in time (T=0), such that:- i) the control unit (20A-J, 55A-J, 56, 58A-B) is capable of permitting fluid communication of hydraulic fluid between the at least one hydraulic fluid line (L1, L2) through the open valve (30A-J) such that it is communicable to the downhole tool (10A-J) during the said window of time (WO1, WO2); and ii) wherein the control unit (20A-J, 55A-J, 56, 58A-B) is further capable of closing the valve (30A-30J) at the end of the window of time (WO1, WO2) determined by the timer (55A-J, 56, 58A-B) such that the control unit (20A-J, 55A-J, 56, 58A-B) prevents hydraulic fluid from flowing through the closed valve (30A-J) and the hydraulic fluid is thereby prevented from acting upon the respective downhole tool (10A-J, 50A-J) at all times outwith the said window of time (WO1, WO2); and i) the control unit (20A-J, 55A-J, 56, 58A-B) is capable of permitting fluid communication of hydraulic fluid between the at least one hydraulic fluid line (L1, L2) through the open valve (30A-J) such that it is communicable to the downhole tool (10A-J) during the said window of time (WO1, WO2); and ii) wherein the control unit (20A-J, 55A-J, 56, 58A-B) is further capable of closing the valve (30A-30J) at the end of the window of time (WO1, WO2) determined by the timer (55A-J, 56, 58A-B) such that the control unit (20A-J, 55A-J, 56, 58A-B) prevents hydraulic fluid from flowing through the closed valve (30A-J) and the hydraulic fluid is thereby prevented from acting upon the respective downhole tool (10A-J, 50A-J) at all times outwith the said window of time (WO1, WO2); and b) controlling the pressure in the at least one hydraulic fluid line (L1, L2) for at least a sufficient period of time required to at least actuate the said downhole tool (10A-10J, 50A-J), wherein the said sufficient period of time coincides at least partially with the said window of time (WO1, WO2) such that the downhole tool (10A-J, 50A-J) is capable of being actuated during the said window of time (WO1, WO2) and is prevented from being actuated at all times outwith the said window of time (WO1, WO2).