The invention provides compounds of formula (I) wherein R4 is a group of formula (II) or (IIIa) or (IIIb) and R1, R2, R3, R5, a, b and X are as defined in the specification, a process for their preparation, pharmaceutical compositions containing them, a process for preparing pharmaceutical compositions, their use in therapy and intermediates of use in their preparation (I), (II), (IIIa), (IIIb).