Compounds of formula (I) or salts thereof, wherein the dotted Line represents a single or double bond, R1 is alkyl or amino optionally substituted by alkyl, alkanoyl or benzyl group; R2, R3, R4 and R5 are the same or different and each is selected from hydrogen, phenyl, halo, nitro, a group S(O)nR8 wherein n is the integer 0, 1 or 2 and R8 is halo or alkyl or a group NR9R10 wherein R9 and R10 are both hydrogen, a group NR11R12 wherein R11 and R12 are the same or different and each is hydrogen or alkyl, a group OR13 wherein R13 is hydrogen or C1-4 of alkyl optionally substituted by halo; a C1-4 aliphatic group optionally substituted by a group OR14 or NR14R15 wherein R14 and R15 are the same or different and each is hydrogen or alkyl; or two of R2 to R5 are linked together to form a benzo group, or one of R2 to R5 is a group X—Y—R16 wherein X is CH2, NR17, CO or S(O)m and Y is CH2, NR17, O or S(O)m, or X—Y is O, NR17, —CH═CH— or N═N—, are disclosed as pharmacological agents useful in the treatment of tumours. Pharmaceutical compositions and methods for the preparation of compounds of formula (I) are also described.