A compound with the formula I: wherein: R 2 is of formula II: where A is a C 5-7 aryl group, X is selected from the group comprising: OH, SH, CO 2 H, COH, N=C=O, NHR N, wherein R N is selected from the group comprising H and C 1-4 alkyl, and (OC 2 H 4) m OCH 3, where m is 1 to 3, and either: (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; R 12 is a C 5-10 aryl group, optionally substituted by one or more substituents selected from the group comprising: halo, nitro, cyano, C 1-7 alkoxy, C 5-20 aryloxy, C 3-20 hetercyclyloxy, C 1-7 alkyl, C 3-7 heterocyclyl and bis-oxy-C 1-3 alkylene, wherein the C 1-7 alkoxy group may be substituted by an amino group, and if the C 3-7 heterocyclyl group is a C 6 nitrogen containing heterocyclyl group, it may be substituted by a C 1-4 alkyl group; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NHRR', nitro, Me 3 Sn and halo; either: (a) R 10 is H, and R 11 is OH, OR A, where R A is C 1-4 alkyl; (b) R 10 and R 11 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 10 is H and R 11 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms,, and/or aromatic rings; Y and Y' are selected from O, S, or NH; R 6', R 7', R 9' are selected from the same groups as R 6, R 7 and R 9 respectively and R 10' and R 11' are the same as R 10 and R 11, wherein if R 11 and R 11' are SO z M, M may represent a divalent pharmaceutically acceptable cation, and wherein the term alkyl includes alkenyl, alkynyl and cycloalkyl and the term aryl includes heteroaryl. R 2 is of formula II: where A is a C 5-7 aryl group, X is selected from the group comprising: OH, SH, CO 2 H, COH, N=C=O, NHR N, wherein R N is selected from the group comprising H and C 1-4 alkyl, and (OC 2 H 4) m OCH 3, where m is 1 to 3, and either: (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; R 12 is a C 5-10 aryl group, optionally substituted by one or more substituents selected from the group comprising: halo, nitro, cyano, C 1-7 alkoxy, C 5-20 aryloxy, C 3-20 hetercyclyloxy, C 1-7 alkyl, C 3-7 heterocyclyl and bis-oxy-C 1-3 alkylene, wherein the C 1-7 alkoxy group may be substituted by an amino group, and if the C 3-7 heterocyclyl group is a C 6 nitrogen containing heterocyclyl group, it may be substituted by a C 1-4 alkyl group; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NHRR', nitro, Me 3 Sn and halo; either: (a) R 10 is H, and R 11 is OH, OR A, where R A is C 1-4 alkyl; (b) R 10 and R 11 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 10 is H and R 11 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; (a) R 10 is H, and R 11 is OH, OR A, where R A is C 1-4 alkyl; (b) R 10 and R 11 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 10 is H and R 11 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms,, and/or aromatic rings; Y and Y' are selected from O, S, or NH; R 6', R 7', R 9' are selected from the same groups as R 6, R 7 and R 9 respectively and R 10' and R 11' are the same as R 10 and R 11, wherein if R 11 and R 11' are SO z M, M may represent a divalent pharmaceutically acceptable cation, and wherein the term alkyl includes alkenyl, alkynyl and cycloalkyl and the term aryl includes heteroaryl.