Welcome to BP.net a lot of people here will be able to help you with this issue,
i have a 10 week old ball that weighs around 86 grams which is pretty light aswell..
but balls can go months without eating with no ill effect...
but im sure more people will weigh in who will be 100% more helpful then me
but welcome to BP.net a fellow UKer!
Also balls are nocturnal so they will roam there cages at night, and sleep through the day.