I've just copied this from my atkins book hope it helps x
"Another problem people occasionally experience is leg cramps at night due to a rapid excretion of calcium, magnesium or potassium. Be sure you are getting enough of these minerals in your multivitamin or take a seperate multimineral supplement. If you are still getting leg cramps you may need to add more."
Also on another page there is:
"Leg Cramps - If you have leg cramps, it probably means you are losing too many electrolytes, which are full of minerals. Starting any weight loss program has a diuretic effect, which is one of the reasons it is so important to stay hydrated. Supplement with potassium, magnesium and calcium in addition to your multivitamin and the cramps should disappear."