1. Amino acid particles in which a sample of the particles has a bulk density not more than 0.1 g/cm<3>. 2. Amino acid particles according to claim 1, in which a sample of the particles has a bulk density not more than 0.05 g/cm<3>. 3. Amino acid particles having a mass median aerodynamic diameter (MMAD) not more than 5 mum. 4. Amino acid particles being in the form of flakes having a thickness of not more than 0.5 mum. 5. Amino acid particles according to claim 5 in which the flakes have a thickness of not more than l00 nm. 6. Amino acid particles according to any preceding claim, in which the amino acid is leucine. 7. A powder for use in a dry powder inhaler, the powder including active material and amino acid particles according to any of claims 1 to 6. 8. A powder according to claim 7, in which the powder includes not more than 20% by weight of amino acid based on the weight of the powder. 9. A power according to claim 8, in which the powder includes not more than 10% by weight of amino acid based on the weight of the powder. 10. A powder according to claim 7 or claim 8, the powder further including particles of a diluent. 11. A powder according to claim 10, in which the diluent includes crystalline sugar. 12. A powder according to claim 10, in which the diluent has a particle size such that at least 90% by weight of the diluent particles have a particle size not more than 10 mum. 13. A powder according to claim 10, or claim 11, in which the diluent has a particle size such that at least 90% by weight of the diluent particles have a particles size not less than 50 mum. 14. A powder according to claim 10 or claim 11, in which the diluent has a fine particle portion having a particle size such that at least 90% by weight of the particle of the fine particle portion have a particle size not more than 10 mum. and a coarse particle portion having a particle size such that at least 90% by weight of the particles of the coarse particle portion have a particle size not less than 50 mum. 15. A powder according to claim 14, in which the fine particle portion and coarse particle portion comprise the same material. 16. A powder according to any of claims 14 or 15, in which, in which the powder includes not more than 5% by weight of the fine particle portion based on the weight of the powder. 17. A powder according to any of claims 14 to 16, in which the powder includes not more than 95% by weight of the coarse particle portion based on the weight of the powder. 18. A dry powder inhaler, the inhaler containing powder according to any of claims 7 to 17. 19. A method of preparing particles of amino acid according to any one of claims 1 to 6, the method including the step of forming solid amino acid particles from a vapour or in a solvent, the method being such that the particles are formed while being suspended in a gas flow. 20. A method of preparing particles of amino acid according to any one of claims 1 to 6, the method including the step of condensing amino acid vapour to form solid amino acid particles. 21. A method according to claim 19 or claim 20, in which particles of amino acid are formed by aerosol condensation. 22. A method according to claim 20 or claim 21, in which the method includes the steps of a) heating the amino acid so that the amino acid forms a vapour; b) mixing the amino acid vapour with cool air to form a cloud of condensed amino acid particles; and c) collecting the condensed particles. 23. A method according to any of claims 20 to 22, the method including the step of heating the amino acid particles to a temperature of at least 150 degree C at ambient pressure. 24. A method of producing particles of amino acid according to any one of claims 1 to 6, in which droplets of amino acid in a solvent are dried in a step of spray drying to form solid particles of amino acid, wherein at least part of amino acid is sublimated after spray drying. 25. A method of producing particles of amino acid comprising amino acid particles according to any one of claims 1 to 6, and particles of active material, in which in which droplets of amino acid in a solvent are dried in the step of spray drying to form solid particles of amino acid, wherein at least part of amino acid is sublimated after spray drying. 26. A method according to claim 24 or claim 25, in which amino acid is condensed together with active material. 27. A method according to claim 25 or claim 26, in which active material is selected from: beta2 agonist, salbutamol and salt thereof, salbutamol sulphate, salmeterol and salt thereof, peptides, polypeptides, DNase, leucotrienes, insulin, terbutaline and salt thereof, terbutaline sulphate, ipatropium bromide, steroid, beclomethasone dipropionate, fluticasone, cromone, sodium cromoglycate, nedocromil or salt thereof, a leukotriene receptor antagonist, a carbohydrate and heparin. 28. A method according to claim 19, in which the method includes the step of spray drying to form solid particles of amino acid. 29. A method according to claim 28, in which the material to be dried comprises amino acid in solution. 30. A method according to any one of claims 24 to 27 or claim 29, in which material to be dried comprises amino acid in aqueous solution. 31. A method according to any of claims 24 to 30, in which the droplets dried have a mean size of not more than 10 mum. 32. A method according to any of claims 19 to 31, in which the method is such that the MMAD of the solid amino acid particles produced is not more than 10 mum. 33. Particles of amino acid obtainable by a method according to any of claims 19 to 32. 34. A method of making a powder according to any of claims 7 to 17, the method including the steps of mixing amino acid according to any of claims 1 to 6 or claim 33 with active material.] 35. A method of making a powder according to any of claims 10 to 17, the method including the step of mixing amino acid according to any of claims 1 to 6 or claim 33 with active material followed by the step of mixing amino acid and active material with a diluent. 36. Use of particles of amino acid according to any of claims 1 to 6 or claim 33 in a powder to improve the flow properties of the powder. 37. Use of particles of amino acid according to any of claims 1 to 6 or claim 33 in a powder for use in a dry powder inhaler.