A compound of formula II and tautomeric forms thereof wherein: R1 - R4 are H, alkyl, aryl, alkaryl, halogen, hydroxy, alkoxy, aryloxy, aralkoxy, acyloxy, amino or alkylamino; R8 has the same meaning as given to R1 - R4 or may be glycosyloxy; R9 and R12 have the same meaning as given to R1 - R4 and alternatively may be alkoxicarbonyl, aryloxycarbonyl or carboxy; one of Rx or Rb is hydroxy and the other is H, hal, alkoxy, aryloxy or acyloxy; R10 and R11 taken together are oxo or ethylenedioxy (provided that if R1 and R11 taken together are ethylenedioxy then if R4 is hydrogen or methoxy. Rb is not hydrogen or acetoxy); or R10 is hydroxy; and R11 is hydroxyalkyl or dihydroxyalkyl or -CO0R15 where R15 is H, alkyl, aryl, aralkyl, hydroxyalkyl.