as defined above; R3 is hydrogen, C1 -C6 alkyl or C2 -C6 alkanoyl; or a pharmaceutically acceptable salt thereof; and wherein: (i) when, at the same time, R2 is hydrogen, C1 -C6 alkyl, halogen or cyano and R3 is hydrogen, R1 and n being as defined above, then at least one of R4, R5 and R6 is other than hydrogen; and (ii) when n is zero and R3 and R6 are each hydrogen or CH3, then at least one of R