1. A skin wash composition comprising from 0.1 to 10% w/w of an .alpha.-hydroxy acid or other acid and a detergent base consisting of a mixture of a non-ionic alkylpolyglucoside surfactant and an amphoteric surfactant wherein the pH of the said composition is in the range 3.0 to 4.5. 2. A composition according to claim 1 wherein the acid is citric acid, malic acid, maleic acid, pyruvic acid, hydroxy-octanoic acid, salicylic acid, lactic acid or glycolic acid. 3. A composition according to claim 2 wherein the acid is salicyclic acid. 4. A composition according to claim 3 comprising from 0.2 to 5.0% w/w salicylic acid. 5. A composition according to claim 4 comprising 1.0 to 3.0% w/w salicylic acid. 6. A composition according to claim 2 wherein the .alpha.-hydroxy acid is lactic acid which comprises from 0.1 to 5.0% w/w of the composition. 7. A composition according to claim 6 comprising from 0.5 to 2.5% w/w lactic acid. 8. A composition according to claim 2 wherein the .alpha.-hydroxy acid is glycolic acid which comprises from 2.0 to 10.0% w/w of the composition. 9. A composition according to any one of claims 1 to 8 wherein the alkylpolyglucoside surfactant is decyl glucoside, lauryl glucoside or mixtures thereof. 10. A composition according to any one of claims 1 to 9 wherein the alkylpolyglucoside surfactant comprises up to 20% w/w of the said composition. 11. A composition according to claim 10 wherein the alkylpolyglucoside surfactant comprises from 2.0 to 15% w/w of the composition. 12. A composition according to any one of claims 1 to 11 wherein the amphoteric surfactant comprises at least one betaine. 13. A composition according to claim 12 wherein the betaine is cocoamidopropyl betaine. 14. A composition according to any one of claims 1 to 13 wherein the amphoteric surfactant comprises up to 10% w/w of the said composition. 15. A composition according to claim 14 wherein the amphoteric surfactant comprises from 2.0 to 8.0% w/w of the composition. 16. A composition according to any one of claims 1 to 15 wherein the detergent base comprises up to 30% w/w of the said composition. 17. A composition according to any one of claims 1 to 16 further comprising a neutralizing agent. 18. A composition according to claim 17 wherein the said neutralizing agent is tromethamine, sodium hydroxide or triethanolamine. 19. A method for the preparation of a composition as defined in any one of claims 1 to 18, which method comprises the step of admixing an .alpha.-hydroxy acid or other acid with a detergent base in an aqueous solvent system and adjusting the pH as required. 20. A composition as defined in any one of claims 3 to 5 and 9 to 18 for use in the treatment and/or prophylaxis of acne. 21. Use of a composition as defined in any one of claims 3 to 5 and 9 to 18 in the manufacture of a medicament for use in the treatment and/or prophylaxis of acne. 22. A method for the treatment and/or prophylaxis of acne which comprises the use of an effective amount of medicament as defined in any one of claims 3 to 5 and 9 to 18. 22. Use of a composition as defined in any one of claims 1 to 18 as a cosmetic treatment for improving the appearance and condition of human skin.