A compound of formula wherein Y 1, Y 2, Y 3 and Y 4 are independently an optionally substituted carbon or a nitrogen atom, wherein the optional substituents are selected from fluoro, chloro, bromo, iodo, and wherein no more than one of Y 1, Y 2, Y 3 and Y 4 is a nitrogen atom; W is a methylene group or CH(alkyl) group substituted by an optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted heterocycloalkyl or optionally substituted heteroaryl moiety, wherein the substituents and optional substituents are selected from halo, hydroxy, cyano, carboxy, C 1-4 alkyl or C 1-4 alkoxy; and X is an optionally substituted aryl, optionally substituted heteroaryl, optionally substituted cycloalkyl, or optionally substituted heterocycloalkyl moiety, which may either be attached directly to the thiadiazine ring or attached through an alkylene linker group of up to three carbon atoms; wherein the optional substituents are selected from fluoro, chloro, bromo, iodo, oxo, C 1-4 alkyl, C 1-4 alkylsulfanyl, C 1-4 alkylsulfinyl, C 1-4 alkylsulphonyl, C 3-6 cycloalkyl, hydroxy, C 1-4 alkoxy, cyano, nitro, amino, C 1-4 alkylamino, C 2-4 alkenylamino, di-C 1-4 alkylamino, C 1-4 acylamino, phenyl, phenylamino, benzoylamino, benzylamino, phenylamido, carboxy, C 1-4 alkoxycarbonyl, phenyl-C 1-10 alkoxycarbonyl, carbamoyl, mono-C 1-4 carbamoyl, di-C 1-4 carbamoyl and any of the above in which a hydrocarbyl moiety is itself substituted by halo, cyano, hydroxy, C 1-2 alkoxy, amino, nitro, carbamoyl, carboxy or C 1-2 alkoxycarbonyl; and wherein either at least one of Y 1 -Y 4 is a nitrogen atom; or X is an optionally substituted heteroaryl, or optionally substituted heterocycloalkyl moiety; or W is a moiety of formula wherein R' and R" are independently H or alkyl and R"' is 2, 4, 6 tri-substituted aryl, or optionally substituted heteroaryl group, wherein the heteroaryl group is a 5 or 6-membered ring containing one or more nitrogen atoms; and wherein the substituents and optional substituents are selected from halo, C 1-4 alkyl and C 1-4 alkoxy. wherein Y 1, Y 2, Y 3 and Y 4 are independently an optionally substituted carbon or a nitrogen atom, wherein the optional substituents are selected from fluoro, chloro, bromo, iodo, and wherein no more than one of Y 1, Y 2, Y 3 and Y 4 is a nitrogen atom; W is a methylene group or CH(alkyl) group substituted by an optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted heterocycloalkyl or optionally substituted heteroaryl moiety, wherein the substituents and optional substituents are selected from halo, hydroxy, cyano, carboxy, C 1-4 alkyl or C 1-4 alkoxy; and X is an optionally substituted aryl, optionally substituted heteroaryl, optionally substituted cycloalkyl, or optionally substituted heterocycloalkyl moiety, which may either be attached directly to the thiadiazine ring or attached through an alkylene linker group of up to three carbon atoms; wherein the optional substituents are selected from fluoro, chloro, bromo, iodo, oxo, C 1-4 alkyl, C 1-4 alkylsulfanyl, C 1-4 alkylsulfinyl, C 1-4 alkylsulphonyl, C 3-6 cycloalkyl, hydroxy, C 1-4 alkoxy, cyano, nitro, amino, C 1-4 alkylamino, C 2-4 alkenylamino, di-C 1-4 alkylamino, C 1-4 acylamino, phenyl, phenylamino, benzoylamino, benzylamino, phenylamido, carboxy, C 1-4 alkoxycarbonyl, phenyl-C 1-10 alkoxycarbonyl, carbamoyl, mono-C 1-4 carbamoyl, di-C 1-4 carbamoyl and any of the above in which a hydrocarbyl moiety is itself substituted by halo, cyano, hydroxy, C 1-2 alkoxy, amino, nitro, carbamoyl, carboxy or C 1-2 alkoxycarbonyl; and wherein either at least one of Y 1 -Y 4 is a nitrogen atom; or X is an optionally substituted heteroaryl, or optionally substituted heterocycloalkyl moiety; or W is a moiety of formula wherein R' and R" are independently H or alkyl and R"' is 2, 4, 6 tri-substituted aryl, or optionally substituted heteroaryl group, wherein the heteroaryl group is a 5 or 6-membered ring containing one or more nitrogen atoms; and wherein the substituents and optional substituents are selected from halo, C 1-4 alkyl and C 1-4 alkoxy.