A compound for use in the treatment of pain, wherein the compound is a compound of the formula: or a salt, solvate, tautomer or N-oxide thereof; wherein the bicyclic group: is selected from the structures C1, C5 and C6: wherein n is 0, 1, 2 or 3; R 1 is hydroxy or hydrogen; R 2a is hydroxy or methoxy (preferably hydroxy) provided that at least one of R 1 and R 2a is hydroxy; R 3 is selected from isopropyl, sec-butyl, tert -butyl, 1,2-dimethylallyl, 1,2-dimethylpropyl and cyclopropyl; R 4a is selected from hydrogen, fluorine, chlorine and methoxy; R 8 is selected from hydrogen and fluorine; and R 10 is selected from: halogen; hydroxy; trifluoromethyl; cyano; nitro; carboxy; amino; mono- or di-C 1-4 hydrocarbylamino; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and a group R a -R b; wherein: R a is a bond, O, CO, X 1 C(X 2), C(X 2)X 1, X 1 C(X 2)X 1, S, SO, SO 2, NR c, SO 2 NR c or NR c SO 2; and R b is selected from hydrogen; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and C 1-12 hydrocarbyl (such as C 1-10 hydrocarbyl) optionally substituted by one or more substituents selected from hydroxy, oxo, halogen, cyano, nitro, carboxy, amino, mono- or di-C 1-8 non-aromatic hydrocarbylamino (e.g. mono- or di-C 1-4 hydrocarbylamino), and carbocyclic and heterocyclic groups having from 3 to 12 ring members and wherein one or more carbon atoms of the C 1-12 hydrocarbyl group (or C 1-10 hydrocarbyl group) may optionally be replaced by O, S, SO, SO 2, NR c, X 1 C(X 2), C(X 2)X 1 or X 1 C(X 2)X 1; R c is selected from R b, hydrogen and C 1-4 hydrocarbyl; and X 1 is O, S or NR c and X 2 is =O, =S or =NR c. wherein n is 0, 1, 2 or 3; R 1 is hydroxy or hydrogen; R 2a is hydroxy or methoxy (preferably hydroxy) provided that at least one of R 1 and R 2a is hydroxy; R 3 is selected from isopropyl, sec-butyl, tert -butyl, 1,2-dimethylallyl, 1,2-dimethylpropyl and cyclopropyl; R 4a is selected from hydrogen, fluorine, chlorine and methoxy; R 8 is selected from hydrogen and fluorine; and R 10 is selected from: halogen; hydroxy; trifluoromethyl; cyano; nitro; carboxy; amino; mono- or di-C 1-4 hydrocarbylamino; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and a group R a -R b; wherein: R a is a bond, O, CO, X 1 C(X 2), C(X 2)X 1, X 1 C(X 2)X 1, S, SO, SO 2, NR c, SO 2 NR c or NR c SO 2; and R b is selected from hydrogen; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and C 1-12 hydrocarbyl (such as C 1-10 hydrocarbyl) optionally substituted by one or more substituents selected from hydroxy, oxo, halogen, cyano, nitro, carboxy, amino, mono- or di-C 1-8 non-aromatic hydrocarbylamino (e.g. mono- or di-C 1-4 hydrocarbylamino), and carbocyclic and heterocyclic groups having from 3 to 12 ring members and wherein one or more carbon atoms of the C 1-12 hydrocarbyl group (or C 1-10 hydrocarbyl group) may optionally be replaced by O, S, SO, SO 2, NR c, X 1 C(X 2), C(X 2)X 1 or X 1 C(X 2)X 1; R c is selected from R b, hydrogen and C 1-4 hydrocarbyl; and X 1 is O, S or NR c and X 2 is =O, =S or =NR c. halogen; hydroxy; trifluoromethyl; cyano; nitro; carboxy; amino; mono- or di-C 1-4 hydrocarbylamino; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and a group R a -R b; wherein: R a is a bond, O, CO, X 1 C(X 2), C(X 2)X 1, X 1 C(X 2)X 1, S, SO, SO 2, NR c, SO 2 NR c or NR c SO 2; and R b is selected from hydrogen; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and C 1-12 hydrocarbyl (such as C 1-10 hydrocarbyl) optionally substituted by one or more substituents selected from hydroxy, oxo, halogen, cyano, nitro, carboxy, amino, mono- or di-C 1-8 non-aromatic hydrocarbylamino (e.g. mono- or di-C 1-4 hydrocarbylamino), and carbocyclic and heterocyclic groups having from 3 to 12 ring members and wherein one or more carbon atoms of the C 1-12 hydrocarbyl group (or C 1-10 hydrocarbyl group) may optionally be replaced by O, S, SO, SO 2, NR c, X 1 C(X 2), C(X 2)X 1 or X 1 C(X 2)X 1; R c is selected from R b, hydrogen and C 1-4 hydrocarbyl; and X 1 is O, S or NR c and X 2 is =O, =S or =NR c. R a is a bond, O, CO, X 1 C(X 2), C(X 2)X 1, X 1 C(X 2)X 1, S, SO, SO 2, NR c, SO 2 NR c or NR c SO 2; and R b is selected from hydrogen; carbocyclic and heterocyclic groups having from 3 to 12 ring members; and C 1-12 hydrocarbyl (such as C 1-10 hydrocarbyl) optionally substituted by one or more substituents selected from hydroxy, oxo, halogen, cyano, nitro, carboxy, amino, mono- or di-C 1-8 non-aromatic hydrocarbylamino (e.g. mono- or di-C 1-4 hydrocarbylamino), and carbocyclic and heterocyclic groups having from 3 to 12 ring members and wherein one or more carbon atoms of the C 1-12 hydrocarbyl group (or C 1-10 hydrocarbyl group) may optionally be replaced by O, S, SO, SO 2, NR c, X 1 C(X 2), C(X 2)X 1 or X 1 C(X 2)X 1; R c is selected from R b, hydrogen and C 1-4 hydrocarbyl; and X 1 is O, S or NR c and X 2 is =O, =S or =NR c.