weight loss plateau

I'm a 21 year old girl at 5"2. My weight is currently 9 stone and 10lbs.

I started dieting this time last year when I stepped on the scales and saw that I had made it to 11 stones!.. I have been on and off dieting since and since I reached 9 stone have not allowed my self to go over 10 stones after that (so I've gone between 9 and 10 stones).

a few weeks ago I reached 9"2 again.. so carried on dieting (eating between 400 and 1000 cals a day) and burning about an average of 700 cals a day at the gym, hoping that I would carry on losing at about 4 lbs per week.. However, my weight loss stalled!.. I actually gained a lb!.. (granted I didn't go to the gym again that week but still walked alot and certainly dieted)..

I gave up and started eating again.. like crazy!.. now i'm as i said 9"10.. i've got 35 days till a big event and would like to lose weight so i'm nicely in the 8 stone range?.. how would anyone suggest i do it without reaching a weight loss plateau this time?.. are there any experts who could guide me?.. i don't mind only losing about a lb a week after the 35 days as i'll have the time and patience then.. Any advice??...

loves Sophie xxx