A compound of formula (I) or a pharmaceutically acceptable salt or solvate thereof, wherein: R2 is of formula (IIa), formula (IIb) or formula (IIc): (a) where A is a C5-7 aryl group, and either (i) Q1 is a single bond, and Q2 is selected from a single bond and -Z-(CH2)n-, where Z is selected from a single bond, O, S and NH and n is from 1 to 3； or (ii) Q1 is -CH=CH-, and Q2 is a single bond； (b) where: RC1, RC2 and RC3 are independently selected from H and unsubstituted C1-2 alkyl； (c) where Q is selected from OH, SH and NRN, and RN is selected from H, methyl and ethyl X is selected from the group comprising: OH, SH, CO2H, COH, N=C=O, NHNH2, CONHNH2, (formula (x)), (formular (y), NHRN, wherein RN is selected from the group comprising H and C1-4 alkyl； and either: when there is a double bond present between C2' and C3', R12 is selected from the group consisting of: (ia) C5-10 aryl group, optionally substituted by one or more substituents selected from the group comprising: halo, nitro, cyano, ether, carboxy, ester, C1-7 alkyl, C3-7 heterocyclyl and bis-oxy-C1-3 alkylene； (ib) C1-5 saturated aliphatic alkyl； (ic) C3-6 saturated cycloalkyl； (id), wherein each of R21, R22 and R23 are independently selected from H, C1-3 saturated alkyl, C2-3 alkenyl, C2-3 alkynyl and cyclopropyl, where the total number of carbon atoms in the R12 group is no more than 5； (ie), wherein one of R25a and R25b is H and the other is selected from: phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy； pyridyl； and thiophenyl； and (if), where R24 is selected from: H； C1-3 saturated alkyl； C2-3 alkenyl； C2-3 alkynyl； cyclopropyl； phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy； pyridyl； and thiophenyl； when there is a single bond present between C2' and C3', R12 is (formula (z)), where R26a and R26b are independently selected from H, F, C1-4 saturated alkyl, C2-3 alkenyl, which alkyl and alkenyl groups are optio
