Yes, you will need to reduce you calorie intake as you lose weight. You should calculate your BMR, there are lots of calculators online just google for one.
Also, maybe your body is just getting to used to having the same amount of calories each day, knows what to expect and therefore manages perfectly (without losing weight) on that. When I did WW a few years ago, there was a popular solution for plateaus like the one you are experiencing. I do not know if you are familiar with WW, but you have points to use every day (which equated to fat and calorie content in food). Say you were on 20 points per day (based on your weight, i.e. linked to BMR), the basic WW diet is to use 20 points each day, day in and day out.
Someone discovered that when they were not losing weight, if they mixed the days up a bit with high and low point days, they started losing again. So over a week they would have say 16-21-18-16-29-16-18, so still averaging 20 points a day.
You could try the same with Slim Fast, having some normal days as you have been doing (making sure you count your calories, use your snacks, eat regularly and drink enough water), but add in some days where you have a lighter evening meal, and use the saved calories for an extra big breakfast and extra snack on another day. Never go under 1000 calories though.
Let me know how you get on if you do try this!
