1. A composition for use in an aerosol inhaler in the form of a solution, the composition comprising an active material, a propellant containing a hydrofluoroalkane (HFA), a cosolvent and further comprising a low volatility component amounting from 0,3 to 10% by weight of the composition to increase the mass median aerodynamic diameter (MMAD) of the aerosol particles on actuation of the inhaler. 2. A composition according to claim 1, wherein the low volatility component has a vapour pressure at 25 degree C not more than 0.1 kPa. 3. A composition according to claim 2, wherein the low volatility component has a vapour pressure at 25 degree C not more than 0.05 kPa. 4. A composition according to any preceding claim, wherein the cosolvent has a vapour pressure at 25 degree C not less than 3 kPa. 5. A composition according to any preceding claim, wherein the cosolvent has a vapour pressure at 25 degree C not less than 5 kPa. 6. A composition according to any preceding claim, wherein the cosolvent is an alcohol. 7. A composition according to any preceding claim, wherein the low volatility component includes a glycol. 8. A composition according to any preceding claim, wherein the low volatility component includes oleic acid. 9. A composition according to any preceding claim, wherein the propellant includes one or more HFAs selected from the group comprising HFA 134a and HFA 227. 10. A composition according to any preceding claim, the composition being such that, on actuation of the aerosol inhaler in use, the MMAD of the aerosol particles is not less than 2 μm. 11. A composition according to any preceding claim, wherein the active material is dipropyonate beclometazone, the cololvent is ethyl alcohol, and the low volatility component is glycerin. 12. Use of a low volatility component in a composition for an aerosol inhaler, the composition comprising an active material, a propellant containing hydrofluoro -alkane (HFA) and a cosolvent, to increase the mass median aerodynamic diameter (MMAD) of the aerosol particles on actuation of the inhaler. 13. Use of a low volatility component according to claim 12 to give a MMAD of the aerosol particles of not less than 2 μm. 14. Use of a low volatility component according to claim 12 or claim 13, wherein the low volatility component has a vapour pressure at 25 degree C not more than 0.1 kPa. 15. Use of a low volatility component according to any of claims 12 to 14, the composition being as claimed in any of claims 1 to 11. 16. An aerosol inhaler containing a composition, the composition being as claimed in any of claims 1 to 11. 17. Method of filling an aerosol inhaler with a composition in the form of solution, the method comprising filling the following components into the inhaler  (a) one or more active materials, (b) one or more low volatility components, amounting from 0,3 to 10% by weight of the composition, (c) one or more cosolvents followed by the addition of a propellant containing a hydrofluoroalkane (HFA). 18. A method according to claim 17, the composition being as claimed in any of claims 1 to 11. 19. Aerosol particles emitted from an aerosol inhaler containing a composition, the composition comprising an active component, a propellant containing a hydrofluoroalkane (HFA), a cosolvent and a low volatility component, wherein the mass median aerodynamic diameter (MMAD) of the aerosol particles is not less than 2 μm. 20. Aerosol particles according to claim 19, wherein the MMAD of the particles is not less than 2.5 μm. 21. Aerosol particles according to claim 19 or claim 20, wherein the composition is according to any of claims 1 to 11.