Abstract: The present invention is concerned with compounds of formula (I) ##STR1## wherein Ar is a mono- or bicyclic aromatic group optionally containing one or two heteroatoms independently selected from nitrogen, oxygen and sulphur, said group being optionally substituted by one or more atoms or groups independently selected from halogen, nitro, amino, --NRR.sup.1 where R and R.sup.1 are independently selected from hydrogen, C.sub.1-8 alkyl and C.sub.1-8 alkanoyl, cyano, carboxyalkoxy, alkoxycarbonylalkoxy, C.sub.1-8 alkyl (including cycloalkyl and cycloalkylalkyl), C.sub.1-8 alkoxy (including cycloalkoxy and cycloalkylalkoxy), C.sub.