A preferred group of compounds are those in which R2 to R6 independently represent hydrogen, R8CO???, NR9R10 CO???, halogen, (C1-6)alkoxy e.g. methoxy. (C1-6)alkylthio e.g. methylthio, or NR9R10 wherein R9 and R10 preferably represent (C1-6)alkyl e.g. dimethylamino, and at least one of R2 to R6 is other than hydrogen: or an adjacent pair of R2 to R6 together with the carbon atoms to which they are