The application concerns a compound of formula I: ##STR1##

wherein one of P and Q is O, and the other of P and Q is CH, where there is a double bond between whichever of Q and P is CH and the carbon atom bearing the R³ group;

• Y is either O or S;
• R¹ and R² are independently hydrogen, an optionally substituted C_1-7 alkyl group, C_3-20 heterocyclyl group, or C_5-20 aryl group, or may together form an optionally substituted heterocyclic ring having from 4 to 8 ring atoms;
• R³ is a phenyl or pyridyl group, attached by a first bridge group selected from —S—, —S(O)—, —S(O)₂—, —O—, —NR^N— and CR^C1R^C2— to an optionally substituted C_5-20 carboaryl group, the phenyl or pyridyl group and optionally substituted C_5-20 carboaryl group being optionally further linked by a second bridge group, so as to form an optionally substituted C_5-7 ring, the phenyl or pyridyl group being further optionally substituted.