More particularly, the phenyl group can be substituted by halogen (e.g. fluorine) or by a C1-2 alkyl group which is substituted by a saturated carbocyclic or heterocyclic group of 3 to 7 ring members and containing up to 2 heteroatoms selected from O, N and S (e.g. a morpholine group).in one embodiment, R5 is an optionally substituted phenyl group and R6 is (i) a saturated carbocyclic or heterocyc