An extendable structure (3, 300, 400) comprising: an outer tube having a threaded region with opposing longitudinal slots therethrough; an inner tube slidable within the outer tube and having a plurality of pairs of transverse holes at spaced apart intervals; a pin (10,134) received through the slots of the outer tube and a pair of opposing transverse holes of the inner tube; a collar mounted on the threaded region and being rotatable to raise and lower the pin (10, 134) within the slots; and a driving member (32, 312); the collar including a driven member (30, 302) mating with the driving member (32, 312), the driving member being connectable to an actuator (37, 38), actuation of the driving member causing rotation of the driven member to effect rotation of the collar, characterised in that: the driven member is a bevel gear (30, 302) extending from a surface of the collar and the driving member is a pinion gear (32, 312) that meshes with the bevel gear provided on the collar, the extendable structure comprising a shaft (34, 134) for connection to the pinion gear, the shaft (34, 134) extending from the pinion gear through a pair of opposing transverse holes in the inner tube, wherein (i) the pin serves as the shaft for connection to the pinion gear and the bevel gear is provided on an upper surface of the collar; or (ii) a separate shaft to the pin serves as the shaft for connection to the pinion gear, the shaft extending from the pinion gear through additional opposing transverse holes provided in the inner tube immediately below the collar, the bevel gear extending from a lower surface of the collar. an outer tube having a threaded region with opposing longitudinal slots therethrough; an inner tube slidable within the outer tube and having a plurality of pairs of transverse holes at spaced apart intervals; a pin (10,134) received through the slots of the outer tube and a pair of opposing transverse holes of the inner tube; a collar mounted on the threaded region and being rotatable to raise and lower the pin (10, 134) within the slots; and a driving member (32, 312); the collar including a driven member (30, 302) mating with the driving member (32, 312), the driving member being connectable to an actuator (37, 38), actuation of the driving member causing rotation of the driven member to effect rotation of the collar, characterised in that: the driven member is a bevel gear (30, 302) extending from a surface of the collar and the driving member is a pinion gear (32, 312) that meshes with the bevel gear provided on the collar, the extendable structure comprising a shaft (34, 134) for connection to the pinion gear, the shaft (34, 134) extending from the pinion gear through a pair of opposing transverse holes in the inner tube, wherein (i) the pin serves as the shaft for connection to the pinion gear and the bevel gear is provided on an upper surface of the collar; or (ii) a separate shaft to the pin serves as the shaft for connection to the pinion gear, the shaft extending from the pinion gear through additional opposing transverse holes provided in the inner tube immediately below the collar, the bevel gear extending from a lower surface of the collar. the driven member is a bevel gear (30, 302) extending from a surface of the collar and the driving member is a pinion gear (32, 312) that meshes with the bevel gear provided on the collar, the extendable structure comprising a shaft (34, 134) for connection to the pinion gear, the shaft (34, 134) extending from the pinion gear through a pair of opposing transverse holes in the inner tube, wherein (i) the pin serves as the shaft for connection to the pinion gear and the bevel gear is provided on an upper surface of the collar; or (ii) a separate shaft to the pin serves as the shaft for connection to the pinion gear, the shaft extending from the pinion gear through additional opposing transverse holes provided in the inner tube immediately below the collar, the bevel gear extending from a lower surface of the collar. (i) the pin serves as the shaft for connection to the pinion gear and the bevel gear is provided on an upper surface of the collar; or (ii) a separate shaft to the pin serves as the shaft for connection to the pinion gear, the shaft extending from the pinion gear through additional opposing transverse holes provided in the inner tube immediately below the collar, the bevel gear extending from a lower surface of the collar.