Intermediates of formula (7) may be prepared by reaction of a compound of formula (8) with an alkylating agent of formula Cy1(Alk1)nZ, where Z is a leaving group such as a halogen atom, e.g. a chlorine, bromine or iodine atom, or a sulphonyloxy group such as an alkylsulphonyloxy, e.g. trifluoromethylsulphonyloxy, or arylsulphonyloxy, e.g. phenylsulphonyloxy, group.