a cyano group; R2 is an alkyl group, especially methyl, optionally substituted with a chlorine atom, a phenyl group, optionally substituted with an alkyl or alkoxy group, or a carboxylic acid or carboxylic acid ester group; or R1 and R2 together with the carbon atoms in the 3- or 4-position of the pyridone ring may form an alicyclic or aromatic ring system so that, for example, R1 and R2 together