The invention provides compounds of formula (I) wherein R 4 is a group of formula (II) or (IIIa) or (IIIb) and R 1 , R 2 , R 3 , R 5 , 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