Today I weighed in 2.25 pounds heavier than yesterday.

Yesterday I ate about 700cals over my limit of quite stodgy food. So I thought I would weigh higher although the weight does tend to drop back over the following couple of days.

Ive never worked out why, when I go over my calorie limit, my weight shoots up the following day and takes 2-3 days to get back on track (but then drops even quicker to catch up with the calorie defecits of said 2-3 days)

Does my body hold waste matter for long periods or just take a long time to digest generally?

It doesn't worry me as all is going well. But I'd like it explained if possible!

"Dodgy" food often is deliciously high in sodium. High sodium consumption causes your body to retain more water in order to maintain the same % sodium in your blood (which is important for all sorts of metabolic processes). Water is very heavy--by volume, heavier than fat (which is why fat floats). Over the next few days, your body flushes out the excess sodium, and with it goes the excess water.

Yeah, what she said. I'm glad you seem to have a level head about it. To me, this was the entire battle. Once I learned and really understood about daily weight fluctuations I was finally able to muster up the patience to see this through.