1. Eletriptan hydrobromide monohydrate of the formula (I): 2. A pharmaceutical composition including eletriptan hydrobromide monohydrate as claimed in claim 1 together with a pharmaceutically acceptable excipient, diluent or carrier. 3. Eletriptan hydrobromide monohydrate as claimed in claim 1 for use as a medicament. 4. The use of eletriptan hydrobromide monohydrate as claimed in claim 1 for the manufacture of a medicament for the treatment of a disease or condition for which a selective agonist of a 5-HT1receptor is indicated. 5. Use as claimed in claim 4 for the treatment of a disease or condition for which a selective agonist of a 5-HT1B/1D receptor is indicated. 6. The use of eletriptan hydrobromide monohydrate as claimed in claim 1 for the manufacture of a medicament for the treatment of a disease or condition selected from migraine, recurrent migraine, hypertension, depression, emesis, anxiety, an eating disorder, obesity, drug abuse, cluster headache, pain, chronic paroxysmal hemicrania and headache associated with a vascular disorder. 7. Use as claimed in claim 6 for the treatment of migraine or recurrent migraine. 8. A method of treatment of a mammal to treat a disease or condition for which a selective agonist of a 5-HT1 receptor is indicated, which includes treating said mammal with an effective amount of eletriptan hydrobromide monohydrate as claimed in claim 1. 9. A method as claimed in claim 8 to treat a disease or condition for which a selective agonist of a 5-HT1B/1D receptor is indicated. 10. A method of treatment of a mammal to treat a disease or condition selected from migraine, recurrent migraine, hypertension, depression, emesis, anxiety, an eating disorder, obesity, drug abuse, cluster headache, pain, chronic paroxysmal hemicrania and headache associated with a vascular disorder which includes treating said mammal with an effective amount of eletriptan hydrobromide monohydrate as claimed in claim 1. 11. A method as claimed in claim 10 to treat migraine or recurrent migraine. 12. A process for the preparation of eletriptan hydrobromide monohydrate as claimed in claim 1 which comprises treatment of a solution of eletriptan in water, or in an organic solvent containing a sufficient amount of water to facilitate formation of the required monohydrate, with hydrogen bromide or a source thereof. 13. A process as claimed in claim 12 wherein the organic solvent is tetrahydrofuran or acetone. 14. A process as claimed in claim 12 or 13 wherein hydrogen bromide is used in the form of an aqueous solution. 15. A process for the preparation of eletriptan hydrobromide monohydrate as claimed in claim 1 which comprises crystallisation of any other form of eletriptan hydrobromide, or a mixture thereof, from water, or from an organic solvent containing a sufficient amount of water to facilitate formation of the required monohydrate. 16. A process as claimed in claim 15 wherein the organic solvent is acetone. 17. A process for the preparation of anhydrous eletriptan hydrobromide which comprises dehydration of any hydrated form of eletriptan hydrobromide, preferably eletriptan hydrobromide monohydrate as claimed in claim 1, or a mixture thereof.