In reaction (a) the radical Q can, for example, be an amino group or a mono- or dialkyl-substituted amino group (each alkyl substituent suitably containing 1 to 4 carbon atoms), hydroxyl, thiol, or an alkoxy, alkylthio or alkylsulphonyl group suitably containing 1 to 4 carbon atoms, for example a methoxy or methylthio group, or a halogen atom, especially a chlorine atom.