First of all i'd recommend making sure you're eating enough in general - as you could just be hungry and therefore wanting bad things because you're not full enough after meals. Second i'd recommend upping your water as even slight dehydration can make you feel hungry and start cravings. Thirdly make sure you're using your syns to their full extent. If you want some crisps, chose a low syn variety and have some. If you want chocolate, chose a small bar but most definately have some! Same goes for anything else. As the others have said if you're constantly denying yourself your fav treats you WILL come off SW at some point. The good thing about this eating plan is you CAN have the things you love and STILL have a healthy diet and lose weight!
If you really think you're going to have a blow out tonight then do it in a controlled way. Still syn what you have and either try to make up for it the rest of the week or have your blip, enjoy it, draw a line under it and carry on sticking to the plan. xx