and R4 is a fluorine, chlorine, bromine or iodine atom; R6 is a straight or branched chain alkyl or alkoxy group containing from 1 to 4 carbon atoms which may be unsubstituted or substituted by one or more halogen atoms which may be the same or different (the trifluoromethyl and trifluoromethoxy groups are preferred), or a chlorine or bromine atom; and R8 is hydrogen or a fluorine, chlorine, bromi