R3 represents (1) a --XH or --X--R7 group, in which X is oxygen or sulphur and R7 is an organic group e.g. an alkyl group including a substituted alkyl group e.g. methyl, a cycloalkyl group including a substituted cycloalkyl group e.g. a cyclohexyl group, an aralkyl group including a substituted aralkyl group e.g. a benzyl group, an aryl group including a substituted aryl group e.g. a phenyl group