Hi Laura,
I know how demoralising it can be to think your efforts aren't working, but they really are. Keep at it!
Don't concentrate too much on 'must lose a stone in 5 weeks' as this sort of thing, while possibly helping you focus, can de-rail you when things don't go exactly to plan.
Two things:
1. Our weight fluctuates daily, and that means up and down. Weighing even every week is sometimes not enough to measure progress as it can take the body longer to reflect changes. If you stick to the plan and under your syn allowance, this will surely show up in your next weigh in.
Things like water retention can affect the scales in the short-term, and while exercise has an effect, it will not drastically affect weight-loss in the short-term (unless done to extremes).
2. No matter what, DO NOT think that the plan is a waste of time because of your STS and fall off the wagon! What you are doing WILL have an effect, as long as you stick with it. Keep in your mind that this is a healthy eating plan for life, and try to focus on the fact that if you stick with the plan 100%, you WILL lose weight after 5 weeks - maybe a stone, maybe not, but it will be a LOSS.
If you don't stick with the plan, chances are in 5 weeks time you may still be the same weight as you are now.
I hope this helps you to stay positive. I have a lifelong problem with being overweight, and have lost and gained weight many times, and I know exactly how it feels to think you've worked hard for nothing, but it really helps to stop thinking about the plan as a quick fix and more as a lifestyle change for good.
Good luck!!