after i had the c-section 10 months ago but the problem is that ive got a bad lower back (due to picking up heavy weight DD) and ive got sore neck (due to not sitting correctly at work) but whenever i do sit ups i get pains in both areas
The best exercise for your tummy muscles is crunches, not sit ups. This describes how to do them. Just make sure you always look at a fixed point on the ceiling when you are doing them (rather than looking where you are going) and you shouldn't involve your neck.
As a long term back sufferer I second the idea of pilates - it is a very safe form of exercise that tries to isolate muscles and make sure you are only working the ones that matter. It is a bit tricky at first because there is a lot to concentrate on, so a good teacher might help. I started with a video (Lynne Robinson with Fern Britten), but eventually found a class. Good luck.