A valve injector comprising: a body defining a fluid conduit; the fluid conduit accommodating first, second and third checks (16, 18, 20); the first check being disposed within the fluid conduit towards a first end of the injector; the second check being disposed towards a second end of the injector, distally from the first end; and the third check being located between said first and second check; wherein each check (16, 18, 20) is biased against a respective first, second and third seat (42, 64, 46) located in the fluid conduit by a respective first, second and third biasing member (40, 66, 62), blocking a respective orifice in the seat and thereby preventing flow through the fluid conduit, wherein a bias force of each of the first, second and third biasing member (40, 66, 62) is such that it is overcomeable by the application of a predetermined pressure in a first direction against the bias force on the respective first, second and third check (16, 18, 20) whereby said check is moved away from its seat and the orifice of the seat is unblocked, wherein upon removal of the pressure, each respective check (16, 18, 20) is operable to return into its position blocking the orifice in the respective seat, wherein the body comprises: a fourth seat longitudinally spaced from the third seat, wherein the injector is characterised in that the fourth seat is located in the fluid conduit, and the body further comprises: a fourth biasing member, and in one mode of operation, the third check is biased against the fourth seat by the fourth biasing member in the direction opposite the bias force of the third biasing member, whereby the third check is retained or compressed between the third and the fourth biasing members and is biased against each of the third and the fourth seats thereby blocking respective orifices in the third and fourth seats; this mode of operation of the injector being a locked mode in which the injector cannot be used for allowing fluid to pass through the fluid conduit. a body defining a fluid conduit; the fluid conduit accommodating first, second and third checks (16, 18, 20); the first check being disposed within the fluid conduit towards a first end of the injector; the second check being disposed towards a second end of the injector, distally from the first end; and the third check being located between said first and second check; wherein each check (16, 18, 20) is biased against a respective first, second and third seat (42, 64, 46) located in the fluid conduit by a respective first, second and third biasing member (40, 66, 62), blocking a respective orifice in the seat and thereby preventing flow through the fluid conduit, wherein a bias force of each of the first, second and third biasing member (40, 66, 62) is such that it is overcomeable by the application of a predetermined pressure in a first direction against the bias force on the respective first, second and third check (16, 18, 20) whereby said check is moved away from its seat and the orifice of the seat is unblocked, wherein upon removal of the pressure, each respective check (16, 18, 20) is operable to return into its position blocking the orifice in the respective seat, wherein the body comprises: a fourth seat longitudinally spaced from the third seat, wherein the injector is characterised in that the fourth seat is located in the fluid conduit, and the body further comprises: a fourth biasing member, and