I dont want to be harsh, but if you know you snack and eat too large a portion, unless you address that, the minute you stop the slimFast the weight will come back, and lets be honest, slimfast is not sustainable long term
Find a smaller plate - modern plates are HUGE, try a charity shop. Eat what you normally eat, only fill 1/3 to half the plate with vegetables and then in the space thats left, add your protein and starch - rice, pasta, potatoes
Get rid of most of the high carb, high fat, high sugar snacks ( and that cereals as well ) and swap them for chicken drumsticks, hard boiled eggs, yoghurt ( greek ) hummus , fruit, cheese, - high protein I say get rid of most, you cant go from one extreme to the other over night, a small portion of what you fancy wont kill. My daily pack of crisp is now every couple of weeks, I have to really really want them