comprises a truncated cone shaped portion 24 between two cylindrical portions 26 and 27 and the outer surface of connecting element 17 fig1 is complementary in shape to the surface of cavity 23 and comprises a truncated cone shaped portion 28 between two cylindrical portions 29 and 30 a seal 31 is provided between cylindrical portion 26 of cavity 23 and cylindrical portion 29 of connecting element 17 to seal the connection between conduit 12 and channel 21 end element 13 also comprises a flange 32 fig3 and 4 in turn comprising a groove defined by two diametrically opposed cavities 33 which are engaged by a snap on fastener indicated as a whole by 34 more specifically fastener 34 comprises a section of elastic material e g music wire is flat and substantially c shaped and comprises two symmetrical arms 36 fig2 shaped to define an arc 37 of over 180 u00b0 and with an inside diameter slightly smaller than that of flange 32 arms 36 comprise respective portions 38 fig4 so shaped as to define a constriction or throat 39 of fastener 34 each portion 38 has a curved preferably arc shaped profile and arms 36 terminate with respective inner bent ends 41 the section of fastener 34 fig1 may have a square or rectangular cross section but is preferably circular in which case the end walls of cavities 33 fig3 have an arc shaped cross section in the plane perpendicular to the axis of conduit 12 cavities 33 fig4 are curved to correspond with the shape of portions 38 cylindrical portion 30 of connecting element 17 comprises an annular groove 42 which in use lies in the same plane as cavities 33 fig1 each cavity 33 is.