There is provided compounds of formula I, wherein R1, R2, R3 R4, R5, R6, R7a and R7b have meanings given in the description, and pharmaceutically-acceptable esters, amides, solvates or salts thereof, which compounds are useful in the treatment of diseases in which inhibition of a protein or lipid kinase (e.g. CDK8 and/or Haspin kinase) is desired and/or required, and particularly in the treatment of cancer or a proliferative disease.