port 408 and an outer side through port 410 a moveable member in the form of a piston 412 is provided in the bore 406 and can move to seal the port 408 similarly a second moveable member in the form of a piston 414 is provided in the bore 406 and can move to seal the port 410 actuators 416 418 control the pistons 412 414 respectively the casing valve sub 400 is run as part of an overall casing string such as a casing string 12 shown in fig1 and positioned such that the port 408 faces an inner annulus and the port 410 faces an outer annulus in use the pistons 412 414 can be moved to different positions as shown in fig4 a 4 b and 4 c by the actuators 416 418 in response to wireless signals which have been received thus the pressure between the inner and outer annuli can be sealed from each other by providing at least one of the pistons 412 414 over or between the respective ports 408 410 as shown in fig4 a 4 c in order to equalize the pressure between the inner and outer annuli the pistons 412 414 are moved to a position outside of the ports 408 410 so they do not block them nor block the bore 406 therebetween as shown in fig4 b the pressures can thus be equalized thus such embodiments can be useful in that they provide an opportunity to equalize pressure between two adjacent casing annuli if one exceeded a safe pressure and or if an emergency situation had occurred the port can then be isolated and pressure monitored.