with a semi circular cross section the grooves 30 a 32 a extend in the direction r in this embodiment the groove 30 a 32 a in the carrier and the mount are positioned such that when the carrier is correctly positioned with respect to the mount the grooves are coaxially aligned and form a conjoined location formation in this embodiment the conjoined location formation is a channel with a circular cross section a seat 34 a is machined on either side of the groove 30 a and the groove 32 a such a machined seat 34 a provides a surface profile and roughness correct for more accurate alignment of the two grooves 30 a and 32 a and consequently more accurate alignment of the carrier with respect to the mount to attach the carrier 12 a to the mount 24 a the carrier is correctly positioned with respect to the mount such that the groove 30 a of the mount is aligned with the groove 32 a of the carrier a locator in this embodiment a dowel not shown in fig3 is then positioned in the conjoined locator channel 31 a the dowel is a close fit to the locator channel 31 a a close fit may equate to a maximum clearance of 0 1 mm the carrier is then attached to the mount using fasteners not shown in fig3 where in this embodiment the fasteners are bolts the bolts screw from underneath the mount with respect to the orientation of the carrier and mount assembly 10 a shown in.