A compound of the formula (I), or a pharmaceutically-acceptable salt, or in-vivo hydrolysable ester thereof: Wherein C is selected from (D) and (E), R2a, R6a, and R3a are independently selected from for example H, CF3, Me and Et; R2b and R6b are independently selected from for example H, F, CF3, Me and Et; R1b is for example optionally substituted diazolyl, triazolyl or tetrazolyl; R4 is for example an optionally substituted 5- or 6-membered heterocyclic ring system. Methods for making compounds of the formula (I), compositions containing them and their use as antibacterial agents are also described.