Would be interesting to know what myths people have about weight loss?

The theory behind your answer is spot on but it depends on a few factors; if your male or female (ill explain later) the rep range you are completing the sit ups too and what type of core exercises you are completing. Males and females adapt to certain exercises differently, for example it is 10 times harder for a woman to put on muscle mass than it is for a man, because muscle mass is built predominately on testosterone and woman use oestrogen during a strength and conditioning workout, so whereas males may increase the size of their muscle, in woman their metabolic rate would improve because without increasing the size of your muscle, your increasing the 'usability' of them. Also with sit ups for woman they would burn a considerable more amount of fat around those muscles working than males because of the way exercise affects woman. Keeping the rep range to at least 12 to no more than 20 would also ensure that the most benefits your feeling are of a fat loss type. The type of core exercise as well could differ results, for example the dreaded plank works mainly our obliques, our obliques when worked help to almost pull our stomach in which for the slimmer waistline is what we are looking for. Now your rep range for the plank is important, if you can easily hold the plank for longer than 40 seconds, try the side plank, again if its too easy for more than 40 seconds, try dropping the hips and bringing them back up again, this will all the time just make sure our body is under sufficient amount of stress to complete an adaptation.
I hope this has helped and im sorry for the long winded reply haha

thanks for that --and fair enough! I am female btw and although i have a pretty toned/usce stomach, the layer of fat that has recently been added desnt want to shift! -dont know what the plank is but sounds like its worth a shot!