reliably in accordance with the present invention by first moulding a nozzle body u2014 the nozzle body in this case being the main casing without the slots u2014 and then post machining the slots 5 into this nozzle body to form the finished main casing the nozzle body 15 is shown in fig6 viewed from the underneath so that the blank underside 7 is clearly visible it may be injection moulded from any suitable plastic such as polycarbonate pc or acrylonitrile butadiene styrene abs or a pc abs blend the elongate slots 5 can be machined on a conventional cnc milling machine using a suitable milling cutter for example a disc cutter may be used to cut the slots 5 fig7 a and 7 b illustrates the resulting slot profile a drawback with the use of a disc cutter is that it produces a radiused end 19 to the slot 5 so that the cross section of the slot 5 u2014 as seen by the airflow u2014 is not uniform it has been found that this can adversely affect the flow structure around the edges of the slot 5 the problem can be reduced somewhat by increasing the disc cutter radius r 1 fig8 b if this is practical but only to a degree in accordance with a further aspect of the invention the problem is solved by using a slot drill to cut the slot 5 with a straight end 21 as shown in fig8 a and 8 b part of the nozzle body 15 can conveniently be moulded with integral bracing elements and then the.