Compounds of the general formula (1) ##STR6## wherein R1 is (a) --AR3 where A is --O-- or --S-- and R3 is phenyl [optionally substituted by C1-4 alkyl, C1-4 alkoxy, C1-4 alkanoyl, methylthio, methylsulphinyl, methylsulphonyl, halogen] --CO2 R4 [where R4 is a hydrogen atom, C1-4 alkyl or phenyl], --NHCOR4, --CONR5 R6 [where R5 and R6 may be the same or different and are each a hydrogen atom or C1-4