Drillstring anchor

A drillstring anchor for reacting torque from a drillstring to a wellbore, the drillstring anchor comprising: a channel for receiving a downhole portion of a drillstring 701 so as to be rotationally engaged therewith; multiple segments 703-710 disposed along the drillstring anchor, each segment comprising a respective gripper 714, 715, 716, 717 and a respective actuator capable of being driven to cause the respective gripper to adopt at least one of (a) a first state in which it is urged outwardly for gripping the wellbore and (b) a second, passive state, the gripper of at least one segment being actuable independently of the gripper of at least one other segment; the multiple segments comprising a first segment and a second segment coupled to each other so as to permit relative longitudinal motion therebetween, figs 7a-7d by, for example, the use of rails 711, 712 and push unit 713.