A linear actuator comprising: a cylinder portion and a base portion ; the cylinder portion comprising a housing defining internally a compression chamber for containing a fluid and comprising a piston assembly that can reciprocate within the compression chamber, said housing further defining a cylinder port in fluid communication with said compression chamber; the base portion defining a first base port and a second base port and a passage extending therebetween fluidly communicating said first base port with said second base port ; in which said cylinder portion is releasably engageable with said base portion such that said cylinder portion and said base portion may be arranged in an engaged condition in which engaged condition said cylinder port and said second base port are in fluid communication; said base portion comprises valve means operable to control the flow of fluid through said second base port , in which said valve means is actuatable between an open condition in which the flow of fluid through the second base port is permitted, and a closed condition in which the flow of fluid through the second base port is substantially prevented; characterised in that said valve means is configured to automatically actuate to said closed condition in which the flow of a fluid across the valve means is substantially prevented when said cylinder portion and said base portion are in a disengaged condition, and in that said cylinder portion comprises an actuating member configured to act upon said valve means in a way to actuate said valve means towards said open condition when said cylinder portion and said base portion are in said engaged condition. a cylinder portion and a base portion ; the cylinder portion comprising a housing defining internally a compression chamber for containing a fluid and comprising a piston assembly that can reciprocate within the compression chamber, said housing further defining a cylinder port in fluid communication with said compression chamber; the base portion defining a first base port and a second base port and a passage extending therebetween fluidly communicating said first base port with said second base port ; in which said cylinder portion is releasably engageable with said base portion such that said cylinder portion and said base portion may be arranged in an engaged condition in which engaged condition said cylinder port and said second base port are in fluid communication; said base portion comprises valve means operable to control the flow of fluid through said second base port , in which said valve means is actuatable between an open condition in which the flow of fluid through the second base port is permitted, and a closed condition in which the flow of fluid through the second base port is substantially prevented; characterised in that said valve means is configured to automatically actuate to said closed condition in which the flow of a fluid across the valve means is substantially prevented when said cylinder portion and said base portion are in a disengaged condition, and in that said cylinder portion comprises an actuating member configured to act upon said valve means in a way to actuate said valve means towards said open condition when said cylinder portion and said base portion are in said engaged condition. <EOS>