An inner tube assembly comprising an inner tube and head assembly and being releasably latchable within a core barrel of a core drilling apparatus, the inner tube assembly comprising: a valve housing having a first leading end and a second trailing end, the housing defining an internal bore extending axially in a direction between the first and second ends; at least one fluid flow port provided through the valve housing to allow fluid communication with a region surrounding the valve housing and the bore; a valve body axially slidable within the bore to open and close the port; a bias member acting on the valve body to bias the valve body in a first axial direction within the bore; and a position guide (33, 34) to guide and control a position of the valve body within the bore relative to the port; wherein the valve body is configured for movement in a second return axial direction within the bore to open and close the port by a fluid pressure acting on the valve body via the second end of the housing, the assembly further comprising a valve pin mounted to be capable of axial movement within the bore to contact the valve body, the bias member configured to bias the pin into contact with the valve body, wherein the position guide (33, 34) comprises a guide slot provided in the housing and a guide pin extending radially from the valve body to be movable within the slot, characterised in that the slot comprises a length having an angled section, the guide pin capable of moving along the length and past the angle section, between a first portion of the guide slot, a second portion (34") and a third portion, which third guide slot portion extends parallel to the longitudinal axial of the head assembly, wherein the guide pin reaching the lower end of slot and being held in this position by fluid pressure, and the angled section comprises a v-shaped profile by the second guide slot (34") being angled upwardly so that the first guide slot and second slot (34") form the v-shape with the lower end at the base of each respective slot, so that continued application of fluid pressure will hold the release valve in this position, and at the same time the release valve keeps the fluid flow ports closed. a valve housing having a first leading end and a second trailing end, the housing defining an internal bore extending axially in a direction between the first and second ends; at least one fluid flow port provided through the valve housing to allow fluid communication with a region surrounding the valve housing and the bore; a valve body axially slidable within the bore to open and close the port; a bias member acting on the valve body to bias the valve body in a first axial direction within the bore; and a position guide (33, 34) to guide and control a position of the valve body within the bore relative to the port; wherein the valve body is configured for movement in a second return axial direction within the bore to open and close the port by a fluid pressure acting on the valve body via the second end of the housing, the assembly further comprising a valve pin mounted to be capable of axial movement within the bore to contact the valve body, the bias member configured to bias the pin into contact with the valve body, wherein the position guide (33, 34) comprises a guide slot provided in the housing and a guide pin extending radially from the valve body to be movable within the slot, characterised in that the slot comprises a length having an angled section, the guide pin capable of moving along the length and past the angle section, between a first portion of the guide slot, a second portion (34") and a third portion, which third guide slot portion extends parallel to the longitudinal axial of the head assembly, wherein the guide pin reaching the lower end of slot and being held in this position by fluid pressure, and the angled section comprises a v-shaped profile by the second guide slot (34") being angled upwardly so that the first guide slot and second slot (34") form the v-shape with the lower end at the base of each respective slot, so that continued application of fluid pressure will hold the release valve in this position, and at the same time the release valve keeps the fluid flow ports closed.