I agree, I plan something in my head but it comes down to what I fancy. I try to keep loads of backup 'free meals' in for times like that, like pasta'n'sauces and tins of Asda vegetable chilli- but when nothing will do it except for what's on your mind, I think you've just got to change the plan and go and get what you want (within reason, I mean if you fancy tuna salad and you'd planned for cottage pie, not if you fancy a Pizza Hut
)
Otherwise I have what I plan, then do exactly what you said and snack and snack and try and curb the fancy- eventually I sometimes end up just going out and getting what I fancied in the first place and eating that AS WELL!! Saying that, I am particularly greedy and when I get a fancy for something I am in trouble
What about planning seven different meals, but not saying 'Monday I will have jacket potato and Tuesday I will have spaghetti Bolognese' instead getting everything you need for seven meals, then each night seeing which one you want the most. That way, you've got a plan in place in that you have the ingredients to have a SW friendly meal, and you know the syns of it because you've thought about it in advance- but you're not stuck with a dinner you don't really have an appetite for.
Does that even make sense?? I say everything in such a roundabout way!!