GUANINE DERIVATIVE

1. Valaciclovir hydrochloride in essentially anhydrous crystalline form including substantially the following d spacing pattern (in Angstroms): 10.20 +- 0.08, 8.10 +- 0.06, 7.27 +- 0.06, 6.08 +- 0.05, 5.83 +- 0.03, 5.37 +- 0.02, 5.23 +- 0.02, 4.89 +- 0.02, 4.42 +- 0.02, 4.06 +- 0.02, 3.71 and 0.02, 3.39 +- 0.02, 3.32 +- 0.02, 2.91 +- 0.02, 2.77 +- 0.02. 2. Crystalline valaciclovir hydrochloride as claimed in Claim 1 having a water of hydration content of not more than 3% w/w. 3. Crystalline valaciclovir hydrochloride as claimed in Claim 2 having a water of hydration content of not more than 2% w/w. 4. Crystalline valaciclovir hydrochloride as claimed in Claim 3 having a water of hydration content of not more than 1% w/w. 5. Crystalline valaciclovir hydrochloride having as claimed in any one the preceding Claims substantially the same X-ray diffraction pattern as shown in Figures 1 to 3. 6. Use of crystalline valaciclovir hydrochloride as a medicament for use in medical therapy. 7. A process for the treatment of herpes viral infection in a human which comprises administering to the human host, an effective anti-herpes viral amount of crystalline valaciclovir hydrochioride as claimed in any one of Claims 1 to 5. 8. A process for the production of valaciclovir hydrochloride in anhydrous crystalline form including substantially the d spacings as defined in Claim 1, said process comprising treating valaciclovir hydrochloride with an amount of 15% to 40% w/w lower alcohol having 1 to 4 carbon atoms or a lower ketone containing from 3 to 6 carbon atoms serving to convert an amount of said valaciclovir hydrochloride into said anhydrous crystalline form; and then isolating said anhydrous crystalline form. 9. A process as claimed in Claim 8, wherein before treating valaciclovir hydrochloride with alcohol or ketone and further said process comprising the steps of forming valaciclovir in solution either in free base or salt form, converting said free base valaciclovir or salt thereof to valaciclovir hydrochioride, isolating valaciclovir hydrochioride from the solution and optionally removing unbound solvent leaving the valaciclovir hydrochioride in substantially dry form. 10. A process as claimed in Claim 9 wherein valaciclovir hydrochloride is treated with ethanol or a solvent composed substantially of ethanol. 11. A process as claimed in Claim 10 wherein ethanol or a solvent composed substantially of ethanol and is added in a range of about 17% by weight to about 40