In another embodiment, R7 is a hydrogen atom, or an unsubstituted of substituted acyl group, for example (1-6C)alkanoyl, such as acetyl or t-butanoyl; benzoyl unsubstituted or substituted by methylenedioxy or one or two substituents selected from (1-4C)alkyl, (1-4C)alkoxy or halogen, such as benzoyl or piperonyl; CONRxRy in which Rx and Ry are each independently hydrogen or (1-4C)alkyl, such as CO