Abstract: A compound of formula (I) or a salt thereof, or a solvate thereof, wherein either (i) Ra represents a group R.sub.5 which is hydrogen, alkyl or optionally substituted aryl and R.sub.5 represents a moiety of formula (a); wherein X represents a hydroxy or an alkoxy group wherein the alkyl group may be substituted or unsubstituted or X represents a group NR.sub.s R.sub.t wherein R.sub.s and R.sub.t each independently represent hydrogen, alkyl, substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted arylalkyl, an optionally substituted heterocyclic group or an optionally substituted heterocyclylalkyl group, or R.sub.s and R.sub.t together may form a heterocyclic group; R.sub.1 represents an alkyl or a substituted or unsubstituted aryl group; and R.sub.2, R.sub.3 and R.sub.4 each independently represent hydrogen, alkyl, aryl or substituted aryl or (ii) R.sub.a represents a moiety of the above defined formula (a) and R.sub.b represents the above defined R.sub.5 ; R.