Weight

Depends how much you weigh right now, how tall you are (to get an idea of whether you are currently underweight, of proper weight, overweight, obese or morbidly obese), how old you are and what your gender is, for starters. Plus how much do you want to lose? 20 kg? If so, then you're keeping it up (1/2 a kg is about a pound) for 40 days and that's an awfully long time.

BTW, I'm a serious weight loser and I've been doing it for over a year and a half, and I only lost 7 pounds in one week twice -- and those weeks were several months apart. You are asking your body to do A LOT. It's not purely impossible but it is tough and not terribly safe unless you're really closely medically supervised. I do not recommend that kind of speed.

So, start here: http://www.nhlbisupport.com/bmi/bmi-m.htm and see where you are before you do anything.

Also, you'll need to watch your caloric intake if you want to lose weight. Do not starve yourself, but you'll have to lay off things like chips (you call them crisps, but also what you call chips, which we call french fries), ice cream, soda, etc. and instead eat things that are good for you, like fruit, vegetables, whole grains, skinless chicken, fish, beef and pork that have had the fat trimmed, nonfat dairy, etc.