There is provided a compound of formula (I) wherein A is selected from --O-- and --S--; X is selected from phenyl optionally substituted with up to 5 substituents each independently selected from halo, C.sub.1-C.sub.4 alkyl and C.sub.1-C.sub.4 alkoxy, thienyl optionally substituted with up to 3 substituents each independently selected from halo and C.sub.1-C.sub.4 alkyl, and C.sub.2-C.sub.8 alkyl, C.sub.2-C.sub.8 alkenyl, C.sub.3-C.sub.8 cycloalkyl and C.sub.4-C.sub.8 cycloalkylalkyl, each of which may be optionally substituted with up to 3 substituents each independently selected from halo, C.sub.1-C.sub.4 alkyl, C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4 alkyl-S(O).sub.n-- where n is 0, 1 or 2, --CF.sub.3, --CN and --CONH.sub.2; Y is selected from phenyl, naphthyl, dihydrobenzothienyl, benzothiazolyl, benzoisothiazolyl, quinolyl, isoquinolyl, naphthyridyl, thienopyridyl, indanyl, 1,3-benzodioxolyl, benzothienyl, indolyl and benzofuranyl, each of which may be optionally substituted with up to 4 or, where possible, up to 5 substituents each independently selected from halo, C.sub.1-C.sub.4 alkyl, C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4 alkyl-S(O).sub.n-- where n is 0, 1 or 2, nitro, acetyl, --CF.sub.3, --SCF.sub.3 and cyano; and when Y is indolyl it may be substituted or further substituted by an N-substituent selected from C.sub.1-C.sub.4 alkyl; Z is selected from OR.sub.3 or F, wherein R.sub.3 is selected from H, C.sub.1-C.sub.6 alkyl and phenyl C.sub.1-C.sub.6 alkyl; R.sub.1 and R.sub.2 are each independently H or C.sub.1-C.sub.4 alkyl; and pharmaceutically acceptable salts thereofwith the proviso that when Y is optionally substituted phenyl or optionally substituted 1,3-benzodioxolyl and Z is OR.sub.3 and X is optionally substituted phenyl then A is --S--.