A mast assembly, comprising: an extendible mast constructed and arranged so as to be configurable between a coiled form and an extended form, wherein when extended the mast is resiliently biased in the form of an elongate tube having a slit along its length defined by longitudinal edges of the mast and wherein when coiled the mast is opened out at the slit to a flattened cross section and wound about an axis extending transversely to the longitudinal extent of the mast; and, at least one slit locking clamp for clamping across the slit in the mast at a discrete position along the length of the mast so as to stabilise one edge relative to the other, wherein the slit locking clamp is separable from the mast wherein clamp is configured as either i) first and second collar members (52,53) pivoted relative to each other, such that they can be opened out to encircle the mast, and a fixture arranged to join the other ends of the collar members to form a collar around the tube, or ii) a first plate spanning the slit external to the wall of the mast and a second plate internal to the wall of the mast and at least partially overlapping with the first plate, and a clamp mechanism extending through the slit and operable to clamp together the walls of the mast between the first and second plates, so as to be, in both clamp configurations (i) and (ii), clampable across the slit at a desired discrete position after the corresponding portion of the mast has been extended, wherein the slit locking clamp has a continuous longitudinal feature or at least two longitudinally separated features for engaging with a corresponding feature (24,90) or features of the mast on one or both sides of the slit to resist movement of one edge relative to the other when clamping the mast. an extendible mast constructed and arranged so as to be configurable between a coiled form and an extended form, wherein when extended the mast is resiliently biased in the form of an elongate tube having a slit along its length defined by longitudinal edges of the mast and wherein when coiled the mast is opened out at the slit to a flattened cross section and wound about an axis extending transversely to the longitudinal extent of the mast; and, at least one slit locking clamp for clamping across the slit in the mast at a discrete position along the length of the mast so as to stabilise one edge relative to the other, wherein the slit locking clamp is separable from the mast wherein clamp is configured as either i) first and second collar members (52,53) pivoted relative to each other, such that they can be opened out to encircle the mast, and a fixture arranged to join the other ends of the collar members to form a collar around the tube, or ii) a first plate spanning the slit external to the wall of the mast and a second plate internal to the wall of the mast and at least partially overlapping with the first plate, and a clamp mechanism extending through the slit and operable to clamp together the walls of the mast between the first and second plates, i) first and second collar members (52,53) pivoted relative to each other, such that they can be opened out to encircle the mast, and a fixture arranged to join the other ends of the collar members to form a collar around the tube, or ii) a first plate spanning the slit external to the wall of the mast and a second plate internal to the wall of the mast and at least partially overlapping with the first plate, and a clamp mechanism extending through the slit and operable to clamp together the walls of the mast between the first and second plates, so as to be, in both clamp configurations (i) and (ii), clampable across the slit at a desired discrete position after the corresponding portion of the mast has been extended, wherein the slit locking clamp has a continuous longitudinal feature or at least two longitudinally separated features for engaging with a corresponding feature (24,90) or features of the mast on one or both sides of the slit to resist movement of one edge relative to the other when clamping the mast.