Hi again Zoe,
OK this is what I've learnt: to lose 1lb of fat you have to create a calorie deficiency of 3500 per week, which is 500 per day. So you'd need to work out how many calories your body burns in a day - I'm sure there are websites which allow you to calculate this by your weight, age and activity level, maybe Google it - and subtract 500 from the figure to lose 1lb a week, or subtract 750 for 1.5lb, 1000 for 2lb (although that'd probably be tough going!)
This is just the scientific basis though, in reality I don't think many people actually lose weight at such a consistent rate!