A compound of the formula ##STR1## wherein X has the formula ##STR2## wherein ring A is phenyl, naphthyl or heterocyclic; wherein R.sup.1 is hydrogen, alkyl, alkanoyl or aroyl; wherein R.sup.2, R.sup.3 and R.sup.4, which may be the same or different, each is an electron withdrawing substituent selected from halogeno, nitro, cyano, trifluoromethyl, alkylthio, alkylsulphinyl and alkylsulphonyl or each is hydrogen, alkyl, alkoxy or dialkylamino provided that when ring A is phenyl or naphthyl at least one of R.sup.2, R.sup.3 and R.sup.4 is an electron-withdrawing substituent; wherein R.sup.5 and R.sup.6, which may be the same or different, each is hydrogen, halogeno or alkyl; wherein R.sup.7 is alkyl or halogenoalkyl; and wherein R.sup.8 is carbamoyl, alkyl, cycloalkyl, alkenyl, alkynyl, halogenoalkyl, halogenoalkenyl, halogenoalkynyl, alkanoyl, alkylcarbamoyl, dialkylcarbamoyl or aroyl; or wherein R.sup.8 is alkyl or alkenyl which bears one or more substituents selected from cyano, carbamoyl, amino, hydroxy, alkanoyl, alkoxy, alkylthio, alkenylthio, alkylsulphinyl, alkenylsulphinyl, alkylsulphonyl, alkenylsulphonyl, alkanoylamino, alkoxycarbonylamino, alkylsulphonamido, alkylamino, dialkylamino, dialkylsulphamoyl, aroyl, aryl, arylthio, arylsulphinyl, arylsulphonyl, heterocyclylthio, heterocyclylsulphinyl and heterocyclylsulphonyl; or wherein R.sup.8 has the formula ##STR3## wherein ring B is phenyl, naphthyl or heterocyclyl and wherein R.sup.2, R.sup.3 and R.sup.4 have any of the meanings stated above, provided that when R.sup.7 is methyl R.sup.8 is not also methyl.