A compound of formula I: and salts and solvates thereof, wherein: 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, NRR', nitro, Me 3 Sn and halo; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, NR N2 (where R N2 is H or C 1-4 alkyl), and/or aromatic rings, e.g. benzene or pyridine; 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; R 11b is selected from OH, OR A, where R A is C 1-4 alkyl; and R L is a linker for connection to a cell binding agent, which is selected from: (iiia): wherein Q is: where Q X is such that Q is an amino-acid residue, a dipeptide residue or a tripeptide residue; X is: where a = 0 to 5, b = 0 to 16, c = 0 or 1, d = 0 to 5; G L is a linker for connecting to a Ligand Unit; and (iiib):, where R L1 and R L2 are independently selected from H and methyl, or together with the carbon atom to which they are bound form a cyclopropylene or cyclobutylene group; and e is 0 or 1; either (a) R 20 is H, and R 21 is OH or OR A, where R A is C 1-4 alkyl; or (b) R 20 and R 21 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 20 is H and R 21 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; or (d) R 20 is H and R 21 is H; or (e) R 21 is OH or OR A, where R A is C 1-4 alkyl and R 20 is selected from: where R Z is selected from: (z-ii) OC(=O)CH 3; (z-iii) NO 2; (z-iv) OMe; (z-v) glucoronide; (z-vi) -C(=O)-X 1 -NHC(=O)X 2 -NH-R ZC, where -C(=O)-X 1 -NH- and -C(=O)-X 2 -NH- represent natural amino acid residues and R ZC is selected from Me, OMe, OCH 2 CH 2 OMe. 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, NRR', nitro, Me 3 Sn and halo; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, NR N2 (where R N2 is H or C 1-4 alkyl), and/or aromatic rings, e.g. benzene or pyridine; 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; R 11b is selected from OH, OR A, where R A is C 1-4 alkyl; and R L is a linker for connection to a cell binding agent, which is selected from: (iiia): wherein Q is: where Q X is such that Q is an amino-acid residue, a dipeptide residue or a tripeptide residue; X is: where a = 0 to 5, b = 0 to 16, c = 0 or 1, d = 0 to 5; G L is a linker for connecting to a Ligand Unit; and (iiib):, where R L1 and R L2 are independently selected from H and methyl, or together with the carbon atom to which they are bound form a cyclopropylene or cyclobutylene group; and e is 0 or 1; either (a) R 20 is H, and R 21 is OH or OR A, where R A is C 1-4 alkyl; or (b) R 20 and R 21 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 20 is H and R 21 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; or (d) R 20 is H and R 21 is H; or (e) R 21 is OH or OR A, where R A is C 1-4 alkyl and R 20 is selected from: where R Z is selected from: (z-ii) OC(=O)CH 3; (z-iii) NO 2; (z-iv) OMe; (z-v) glucoronide; (z-vi) -C(=O)-X 1 -NHC(=O)X 2 -NH-R ZC, where -C(=O)-X 1 -NH- and -C(=O)-X 2 -NH- represent natural amino acid residues and R ZC is selected from Me, OMe, OCH 2 CH 2 OMe. (a) R 20 is H, and R 21 is OH or OR A, where R A is C 1-4 alkyl; or (b) R 20 and R 21 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 20 is H and R 21 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; or (d) R 20 is H and R 21 is H; or (e) R 21 is OH or OR A, where R A is C 1-4 alkyl and R 20 is selected from: where R Z is selected from: (z-ii) OC(=O)CH 3; (z-iii) NO 2; (z-iv) OMe; (z-v) glucoronide; (z-ii) OC(=O)CH 3; (z-iii) NO 2; (z-iv) OMe; (z-v) glucoronide;