A compound of formula (I) and pharmaceutically acceptable salts, solvates and hydrolysable esters thereof ##STR00001## Wherein X.sub.1 represents O or S; R.sup.1 and R.sup.2 are independently H or C.sub.1-3 alkyl or R.sup.1 and R.sup.2 which are bonded to the same carbon atom may together with the carbon atom to which they are bonded form a 3 5 membered cycloalkyl ring; R.sup.3 and R.sup.4 independently represent H, halogen, --CH.sub.3 and --OCH.sub.3; R.sup.5 represents H or C.sub.1-6 alkyl X.sub.2 represents NH, NCH.sub.3 or O; One of Y and Z is N, and the other is O or S; R.sup.6 represents phenyl or pyridyl (wherein the N is in position 2 or 3) and is optionally substituted by one or more halogen, CF.sub.3, C.sub.1-6 straight or branched alkyl (optionally substituted by halogen), with the provision that when R.sup.6 is pyridyl, the N is unsubstituted.