Hi Chrissie, I agree with Elasticwoman. The best things you can do are increase your activity level and cut down on food. It's not necessary to go on a strict (and usually very depressing) diet.
It's a good idea to make small changes on a week-by-week basis, rather than trying to drastically change your lifestyle all in one go.
To benefit from exercise, you need to make sure that you get out of breath and feel warmer - this is important for burning calories. Doing gentle stretching, pilates etc won't burn enough calories to make a difference, though obviously would benefit you in other ways (improving flexibility or strengthening core muscles). You don't need to exercise for hours on end, you can do it in short bursts - for example, if you did 2 15-mins brisk walks a day, that would add up to 2.5 hours of exercise a week.
As far as the eating goes, in my experience strict low-calorie diets are hard to stick to and lower your basic metabolic rate, which means your body burns less calories than before. It would be better to replace regular snacks of things like biscuits with fruit or low-fat yogurt. Another easy way to consume fewer calories is to use a tea-plate rather than a dinner plate to serve your food on. Any bowls of food, use a smaller bowl than what you currently use. Automatically your serving size is reduced. And don't have seconds!
Another easy way to fit in a bit of exercise is to use a DVD - or even just play some loud music and really dance your heart out for 15 mins!
Researchers have found that slow and steady weight loss is more likely to stay off than crash dieting. It's probably better not to keep weighing yourself; weight can fluctuate enormously and you can get really discouraged by seemingly inexplicable variations. It's usually more encouraging to go by how your clothes fit. Anyway, good luck and hope it goes well.