A manual actuating means comprises a pair of lead screw and nut devices (8, 9) mounted within a common housing (10, 11). The housing (10, 11) includes a carrier member (5) containing a gear wheel shaft (6). The gear wheel shaft (6) is coupled to a control knob (3) by means of a shaft (4), and a coaxial shaft (2) couples a further rotary knob (1) to the carrier (5). Rotary movement of the knob (1) causes axial translatory movement of the carrier (5) to bring respective gear wheel shaft (6) into driving engagement with one or other of the lead screws (9) via corresponding bevel gears. Rotary movement of the control knob (3) can thus effect actuation of one or other of the lead screw and nut devices in accordance with the position of the rotary knob (1).