Hello, fellow norn ironer... just outside Belfast myself.
You can get a bit enslaved by the scales. Just a tiny movement can mean elation or despair - it's a bit crazy when you think about it, isn't it? In the early days of my weight loss I didn't weigh myself at all. That was partly cowardice (didn't want to see how heavy I was) but I also knew I could get derailed quickly if I kept watching the scales, so I vowed: no weighing. Didn't weigh for almost a year! But I did use a tape measure and it was encouraging. Maybe go by your measurements instead? Weight is too fickle. Water retention, meal times, posture... all can affect the reading and slowly drive you bananas! If you simply *must* weigh, just do it once a fortnight at most - once a month would be even better.
The thing with restrictive and pretty barmy diets like slimfast, is that you put so much energy into depriving yourself during the week, that if you don't see fast results, you can be so depressed that you dive headlong into the chocolate. And often you won't see quick results just because the body has such an almighty shock to the system that it hoards its fat, rather than letting go of it.
It's totally your decision, but I'd dump the slimfast. Right into the bin. You can't live on the stuff for the rest of your life, so it's not sustainable. When you get to your goal weight, you're going to be eating real food, right? Better to get to grips with handling real food right now than waiting until then and just falling back into old habits. Real food is more satisfying, too! And I know it may not seem like it, but it's the real 'fast' option. If you take one step forward and two steps back with slimfast, it's actually the slow option, not to mention tortuous! Steady weight loss with sensible meals gives you a plan you can stick at for the long-term, and you're more likely to keep the weight off once you get to your goal.
It doesn't have to be about total punishment! Nobody can handle that, after all, you've got to be kind yourself and eat sensibly... I really do think it's the only thing that works in the long term.