hanger 132 and a nut 158 which is screwed down onto the seal 154 to set the seal thus in the embodiment shown in fig5 the invention is used in a drilling rather than a tie back situation in fig7 the equipment is in principle generally the same as in fig5 and 6 although in fig7 an inner casing string 202 has also been run but the adjustment of the casing head 214 is performed not by rotation as in fig5 and 6 but by hydraulic pressure as will now be described fig7 shows the casing head 214 in its final position in engagement with the 133 8 inch casing hanger 208 but after installation of the 133 8 inch casing 204 on a landing shoulder on the 20 inch casing lower in the well the shoulder 212 of the casing head 214 is spaced below the shoulder 210 of the casing hanger 208 and requires to be adjusted to meet it at that stage a lock nut 262 is located at a lower end of its screw thread connection with the outer face of the 20 inch casing 202 with a low pressure bop stack installed on the top of the casing head 214 in the drawing the bop stack has been replaced following installation of the casing strings with the wellhead housing 220 as the intention is to pack off the casing annulus between the 20.