A compound of the following formula: or a pharmaceutically acceptable salt thereof, wherein: R 1 is a 'C-linked' 5- or 6-membered heteroaryl comprising (a) one or two nitrogen atoms or, when 5-membered, (b) one or two nitrogen atoms and one sulphur atom, said heteroaryl being optionally substituted on a ring carbon atom by F or Cl; R 2, R 3 and R 4 are independently H, F, Cl or -OCH 3; R 5 is CN, F, Cl or R 6; R a is phenyl optionally substituted by one to three substituents independently selected from F, Cl or R 6; R b is H, F, Cl or R 6; R 6 is (C 1 -C 4)alkyl or (C 1 -C 4)alkyloxy, each optionally substituted by one to three F; Z 1 is (a) phenyl, optionally substituted by one to three substituents independently selected from F, Cl or R 6; or (b) a 'C-linked' 5- or 6-membered heteroaryl comprising one or two nitrogen atoms, said heteroaryl being optionally substituted by R 7 or R 8, or both R 7 and R 8; R 7 is attached to a Z 1 ring carbon and is selected from F, Cl, NR 9 R 10, R 6, (C 3 -C 8)cycloalkyl or Het 1; R 8 is attached to a Z 1 ring nitrogen and is from (C 1 -C 4)alkyl, (C 3 -C 8)cycloalkyl or 'C-linked' Het 1; Het 1 is a 3- to 8-membered saturated monoheterocycloalkyl comprising one or two ring members selected from -NR 11 - and -O-, said monoheterocycloalkyl being optionally substituted on a ring carbon atom by one to three substituents independently selected from F, (C 1 -C 6)alkyl, (C 1 -C 4)alkyloxy(C 0 -C 4)alkylene and (C 3 -C 8)cycloalky; and R 9, R 10 and R 11 are independently selected from H, (C 1 -C 6)alkyl or (C 3 -C 8)cycloalkyl; or, when Het 1 is 'N-linked', R 11 is absent from that nitrogen atom. R 1 is a 'C-linked' 5- or 6-membered heteroaryl comprising (a) one or two nitrogen atoms or, when 5-membered, (b) one or two nitrogen atoms and one sulphur atom, said heteroaryl being optionally substituted on a ring carbon atom by F or Cl; R 2, R 3 and R 4 are independently H, F, Cl or -OCH 3; R 5 is CN, F, Cl or R 6; R a is phenyl optionally substituted by one to three substituents independently selected from F, Cl or R 6; R b is H, F, Cl or R 6; R 6 is (C 1 -C 4)alkyl or (C 1 -C 4)alkyloxy, each optionally substituted by one to three F; Z 1 is (a) phenyl, optionally substituted by one to three substituents independently selected from F, Cl or R 6; or (b) a 'C-linked' 5- or 6-membered heteroaryl comprising one or two nitrogen atoms, said heteroaryl being optionally substituted by R 7 or R 8, or both R 7 and R 8; R 7 is attached to a Z 1 ring carbon and is selected from F, Cl, NR 9 R 10, R 6, (C 3 -C 8)cycloalkyl or Het 1; R 8 is attached to a Z 1 ring nitrogen and is from (C 1 -C 4)alkyl, (C 3 -C 8)cycloalkyl or 'C-linked' Het 1; Het 1 is a 3- to 8-membered saturated monoheterocycloalkyl comprising one or two ring members selected from -NR 11 - and -O-, said monoheterocycloalkyl being optionally substituted on a ring carbon atom by one to three substituents independently selected from F, (C 1 -C 6)alkyl, (C 1 -C 4)alkyloxy(C 0 -C 4)alkylene and (C 3 -C 8)cycloalky; and R 9, R 10 and R 11 are independently selected from H, (C 1 -C 6)alkyl or (C 3 -C 8)cycloalkyl; or, when Het 1 is 'N-linked', R 11 is absent from that nitrogen atom.