1. An aqueous pharmaceutical composition comprising from 5 to 200 mg/ml of eletriptan hemisulphate and from 0.5 to 2.0% weight/volume of caffeine. 2. A composition as claimed in Claim 1 comprising from 40 to 160 mg/ml of eletriptan hemisulphate. 3. A composition as claimed in Claim 1 or 2 comprising from 60 to 120 mg/ml of eletriptan hemisulphate. 4. A composition as claimed in any of Claims 1-3 comprising from 1.0 to 2.0% weight/volume of caffeine. 5. A composition as claimed in any of Claims 1-4 further comprising an anti-oxidant. 6. A composition as claimed in Claim 5 wherein the anti-oxidant is citric acid. 7. A composition as claimed in Claim 6 wherein up to and including 1.0% weight/volume of citric acid is present. 8. A composition as claimed in Claim 7 wherein from 0.2 to 0.4% weight/volume of citric acid is present. 9. A composition as claimed in Claim 5 wherein the anti-oxidant is ascorbic acid. 10. A composition as claimed in any of Claims 1-9 further comprising ethanol. 11. A composition as claimed in Claim 10 wherein up to and including 20.0% weight/volume of ethanol is present. 12. A composition as claimed in Claim 11 wherein from 2.0 to 10.0% weight/volume of ethanol is present. 13. A composition as claimed in Claim 12 wherein from 2.0 to 6.0% weight/volume of ethanol is present. 14. A composition as claimed in any of Claims 1-13 that is buffered to a pH of from 4.0 to 9.0. 15. A composition as claimed in Claim 14 that is buffered to a pH of from 7.5 to 8.5. 16. A composition as claimed in Claim 14 that is buffered to a pH of from 4.0 to 5.0. 17. A composition as claimed in Claim 1 comprising 120 mg/ml of eletriptan hemisulphate, 1.5% weight/volume of caffeine, 0.3% weight/volume of citric acid and 5% weight/volume of ethanol, with the composition buffered to from pH 7.5 to 8.5, preferably up to 8. 18. A composition as claimed in Claim 17 wherein it is buffered using tris(hydroxymethyl)methylamine and sodium hydroxide. 19. A use of composition according to any of Claims 1-18 as a medicine. 20. A use of composition according to any of Claims 1-18 for preparation a medicine for treatment a disease or condition for which a selective agonist of 5-HT1-receptors is indicated. 21. A use of composition according to any of Claims 1-18 for preparation a medicine for treatment a disease or condition selected from the group consisting of migraine, hypertension, depression, emesis, anxiety, an eating disorder, obesity, drug abuse, cluster headache, pain, chronic paroxysmal hemicrania and headache associated with a vascular disorder. 22. A method as claimed in Claim 21 for treating migraine. 23. A method for treating a human, which is a treatment of a disease or condition for which a selective agonist of 5-HT1-receptors is indicated, wherein effective amount of composition as claimed in any of Claims 1-18 is administered to a human in need. 24. A method for treating a human, which is a treatment of a disease or condition selected from the group consisting of migraine, hypertension, depression, emesis, anxiety, an eating disorder, obesity, drug abuse, cluster headache, pain, chronic paroxysmal hemicrania and headache associated with a vascular disorder, wherein effective amount of composition as claimed in any of Claims 1-18 is administered to a human in need. 25. A method as claimed in Claim 24 for treating migraine.