1. Matrix tablet for the prolonged release ofgliclazide, characterised in that it comprises at least the combination of a cellulose polymer compound and a glucose syrup, that combination enabling control of the prolonged release of gliclazide and enabling insensitivity of the dissolution kinetics ofgliclazide to variations in pH. 2. Gliclazide matrix tablet according to claim 1, characterised in that the cellulose polymer compound comprises at least one hydroxypropyi methylcellulose. 3. Gliclazide matrix tablet according to either claim 1 or claim 2, characterised in that the cellulose polymer compound comprises a mixture of two hydroxypropyi methylcelluloses of different viscosity. 4. Gliclazide matrix tablet according to any one of claims 1 to 3, characterised in that the cellulose polymer compound comprises a mixture of hydroxypropyi methylcellulose of viscosity 4000 cP and hydroxypropyi methylcellulose of viscosity 100 cP. 5. Gliclazide matrix tablet according to claim 1, characterised in that the glucose syrup is maltodextrin. 6. Gliclazide matrix tablet according to any one of claims 1 to 4, characterised in that the percentage of cellulose polymer compound is from 10 to 40 % of the total weight of the said tablet. 7. Gliclazide matrix tablet according to any one of claims 1, 2, 3, 4 and 6, characterised in that the percentage of cellulose polymer compound is from 16 to 26 % of the total weight of the said tablet. 8. Gliclazide matrix tablet according to either claim 1 or claim 5, characterised in that the percentage of glucose syrup is from 2 to 20 % of the total weight of the said tablet. 9. Gliclazide matrix tablet according to any one of claims 1, 5 and 8, characterised in that the percentage of glucose syrup is from 4 to 10 % of the total weight of the said tablet. 10. Gliclazide matrix tablet according to claim 1, characterised in that calcium hydrogen phosphate dihydrate is used as diluent. 11. Gliclazide matrix tablet according to either claim 1 or claim 10, characterised in that the percentage of diluent is from 35 to 75 % of the total weight of the said tablet. 12. Gliclazide matrix tablet according to any one of claims 1, 10 and 11, characterised in that the percentage of diluent is from 45 to 60 % of the total weight of the said tablet. 13. Gliclazide matrix tablet according to claim 1, characterised in that the amount of gliclazide is from 12 to 40 % of the total weight of the said tablet. 14. Gliclazide matrix tablet according to either claim 1 or claim 13, characterised in that it contains a total amount of gliclazide of 30 mg. 15. Gliclazide matrix tablet according to either claim 1 or claim 13, characterised in that it contains a total amount of gliclazide of 60 mg. 16. Gliclazide matrix tablet according to claim 1, characterised in that the percentages of cellulose polymer compound and glucose syrup make possible a constant gliclazide release profile for a dissolution medium pH ranging from 6 to 8. 17. Gliclazide matrix tablet according to claim 1, characterised in that the percentages of cellulose polymer compound and of glucose syrup make possible the release of 50% of the total amount of gliclazide by a time of from 4 to 6 hours. 18. Gliclazide matrix tablet according to claim 1, characterised in that the percentages of cellulose polymer compound and of glucose syrup enable prolonged release of gliclazide that results in humans in blood levels of from 400 to 700 ng/ml 12 hours at most after a single administration of the tablet by the oral route. 19. Process for the preparation of a matrix tablet according to claim 1. characterised in that there are used both a wet granulation technique and a direct compression technique. comprising the following steps: STEP A: Mixture of gliclazide, maltodextrin and calcium hydrogen phosphate dihydrate, followed by wetting of that mixture with purified water. The resulting wet mass is then granulated, dried and subsequently classified to obtain a granulate having physical characteristics that enable good filling of the moulds of a rapid-compression machine. STEP B: Mixture of the granulate obtained in Step A with hydroxypropyi methylcellulose. STEP C: Lubrication of the mixture obtained in Step B with colloidal silica and magnesium stearate. STEP D: Compression of the lubricated mixture obtained in Step C using a rotary compression machine to obtain tablets having a hardness, measured by diametric crushing, of about from 6 to 10 daN. 20. Gliclazide matrix tablet according to claim 1 for use in the treatment of diabetes.