During this study the participant will be seen in clinic on a regular basis; initially every 2 weeks, later once a month. There will be a total of 21 clinic visits during the study. Three of these are clinic visits that would normally take place in the routine management of this disease. The other 18 visits are required for this study. At each visit participants will have blood tests to measure the amount of magnesium in the blood, especially the red blood cells, but also to look for any other changes that might occur. The total amount of blood collected during the 16 month study is 120 ml (24 teaspoons), which is a safe amount. Three ml (half teaspoon) will be drawn at 13 visits, 5ml (1 teaspoon) at 4 visits, and 15ml (3 teaspoons) at a further 4 visits. At 4 of the clinic visits a urine test will be performed.