pressure produces an axial force on the sleeve 14 tending to move the sleeve 14 in direction u201c a u201d to align the flow ports 16 18 such movement of the sleeve 14 relative to the body 12 is initially resisted and restrained by a latch arrangement 44 comprising a latch key 46 located in an aperture in a spring mounting latch sleeve 48 and a body shoulder 50 and a sleeve shoulder 52 defined by the body 12 and the sleeve 14 respectively initially as illustrated in fig1 the latch key 46 sits on the body shoulder 50 such that axial movement of the sleeve 14 tending to move the sleeve 14 towards the position where the ports 16 18 are aligned will bring the sleeve shoulder 52 into contact with the key 46 and the sleeve 14 will thus be axially restrained by the key 46 the spring mounting latch sleeve 48 defines three spring stops 54 56 58 each engaging a respective spring 60 62 64 the body 12 is provided with corresponding spring stops 66 68 70 of decreasing diameter by appropriate selection of springs the trip pressure for the apparatus 10 may be between 750 3500 psi in this example 3500 psi and if desired only one or two springs may be provided in use a positive differential pressure between the string interior and surrounding annulus results in application of an axial force to the sleeve 14 in direction u201c a u201d as noted above the sleeve 14 is initially restrained by the contact between the sleeve shoulder 52 and the latch key 46 however on the pressure differential reaching a sufficient level the u201c cracking u201d pressure the resulting force applied to the key 46 by.