A compound of formula (I): ##STR00001## wherein X represents O or S; R.sub.1 represents C.sub.1-6 alkyl, C.sub.3-8 cycloalkyl, C.sub.3-8 cycloalkylmethyl or C.sub.3-8 cycloalkenyl any of which optionally may be substituted by one or more methyl groups or halogen atoms or R.sub.1 represents aryl, substituted aryl, heteroaryl or substituted heteroaryl; R.sub.2 represents hydrogen, methyl, which may be in either the .alpha. or .beta. configuration, or methylene; R.sub.3 and R.sub.4 are the same or different and each independently represents hydrogen, halogen or a methyl group; and represents a single or a double bond; or a physiologically acceptable salt or solvate thereof.