1. A use of liquid composition as a tooth erosion inhibitor containing a calcium compound and an acidulant, characterised in that calcium is present in the range of 0.3 to 0.8 mol per mol of acid and pH of the composition is from 3.5 to 4.5; excluding a fruit-containing liquid foodstuffs having a fruit content of 25 to 100% by weight and an acid content of at least 5g/litre (calculated as tartaric acid), and containing at least 2000 mg/litre of dissolved calcium phosphate. 2. A use as claimed in Claim 1, wherein the calcium is present in the range of 0.3 to 0.55 mol per mol of acid. 3. A use as claimed in Claim 1 or 2, wherein the calcium is present in the amount of at least 0.4 mol per mol of acid. 4. A use as claimed in Claim 1, 2 or 3 in which the pH of the composition is not more than 4. 5. A use as claimed in any one of Claims 1-4 in which the pH is from 3.7 to 3.9. 6. A use as claimed in any one of Claims 1 to 5 in which is an acidic liquid composition is a beverage. 7. A use as claimed in Claim 6 in which the beverage is a still fruit drink, or a carbonated soft drink or a health drink. 8. A use as claimed in any one of Claims 1 to 5 in which is an acidic liquid composition is a drink concentrate for the preparation of a beverage. 9. A use as claimed in Claim 8 wherein a concentrate is intended for a fruit drink or health drink. 10. A process for preparing a liquid composition, containing a calcium compound and an acidulant, characterised in that calcium is present in the range 0.3 to 0.8 mol per mol of acid and the pH of the composition is from 3.5 to 4.5, comprising mixing the calcium compound with the acidulant so that calcium is present in the range of 0.3 to 0.8 mol per mol of acid, adjusting the pH if it is necessary or desired by addition of an alkali so that the pH of the composition is in the range 3.5 to 4.5; excluding a fruit-containing liquid foodstuffs having a fruit content of 25 to 100% by weight and an acid content of at least 5g/litre (calculated as tartaric acid), and containing at least 2000 mg/litre of dissolved calcium phosphate. 11. A method as claimed in Claim 10, wherein the calcium is present in the beverage in the range of 0.3 to 0.55 mol per mol of acid. 12. A method as claimed in Claim 10 or 11, in which the beverage is a still fruit drink, or a carbonated soft drink or a health drink. 13. A process for preparing a concentrate for preparing a drink, containing a calcium compound and an acidulant, characterised in that calcium is present in the range 0.3 to 0.55 mol per mol of acid and the pH of the concentrate is from 3.5 to 4.5, comprising mixing the calcium compound with the acidulant so that calcium is present in the range of 0.3 to 0.55 mol per mol of acid, adjusting the pH if it is necessary or desired by addition of an alkali so that the pH of the concentrate is in the range 3.5 to 4.5; excluding a fruit-containing liquid foodstuffs having a fruit content of 25 to 100% by weight and an acid content of at least 5g/litre (calculated as tartaric acid), and containing at least 2000 mg/litre of dissolved calcium phosphate. 14. A method as claimed in Claim 13 in which the concentrate is for a fruit drink or health drink. 15. A method of reducing the tooth erosion properties of an acidic oral composition comprising adding calcium to the acidic liquid oral composition so that calcium is present in the range of 0.3 to 0.8 mol per mol of acid and the pH of the composition is from 3.5 to 4.5, adjusting the pH if it is necessary or desired by addition of an alkali so that the pH is in the range 3.5 to 4.5; excluding a fruit-containing liquid foodstuffs having a fruit content of 25 to 100% by weight and an acid content of at least 5g/litre (calculated as tartaric acid), and containing at least 2000 mg/litre of dissolved calcium phosphate. 16. A method as claimed in Claim 15 in which calcium is present in a composition in the range of 0.3 to 0.55 mol per mol of acid. 17. The use of a liquid composition comprising a calcium compound and an acidulant in the manufacture of a medicament for the reduction of tooth erosion caused by acid in orally administered compositions, characterized in that the calcium is present in the range 0.3 to 0.8 mol per mol of acid and that the amount of calcium and acidulant in the composition is selected so that the pH of the composition is from 3.5 to 4.5; excluding a fruit-containing liquid foodstuffs having a fruit content of 25 to 100% by weight and an acid content of at least 5g/litre (calculated as tartaric acid), and containing at least 2000 mg/litre of dissolved calcium phosphate. 18. A use as claimed in Claim 17, wherein the calcium is present in a composition in the range of 0.3 to 0.55 mol per mol of acid. 19. A method of reducing the tooth erosion caused by acid in orally administered compositions, characterized in that liquid composition is administered orally, which composition comprises calcium and an acidulant, characterized in that calcium is present in the range of 0.3 to 0.8 mol per mol of acid and the pH of the composition is from 3.5 to 4.5, excluding a fruit-containing liquid foodstuffs having a fruit content of 25 to 100% by weight and an acid content of at least 5g/litre (calculated as tartaric acid), and containing at least 2000 mg/litre of dissolved calcium phosphate. 20. A method as claimed in Claim 19 in which calcium is present in a composition in the range of 0.3 to 0.55 mol per mol of acid.