Dont worry so much about the amount of fat that your having, you can have a virtually fat free diet on Xenical. Although Xenical works by getting rid of the fat you eat, it also works to stop people cheating and eating what they shouldnt (thats how it works for me anyway! lol)
But as Ali said, its very important that you eat no less than 1200 cals a day. Your body goes into starvation mode and will store all your excess fat rather than burning it. Your body thinks its not being fed properly so will keep hold of everything it can. My Dr told me to have no less than 1000 cals a day. But i wasn't losing much weight, about 1lb a week with a couple of weeks at .5lb. When i bumped it to between 1200-1500 i was losing 3-4lbs a week. Also, its worth remembering that Dr's can't know everything, thats why we have dieticians. And also, as Ali said, what on earth are you going to be eating when you need to reduce your calories? As you lose weight, you'll need to reduce your cals but an 8st woman and a 20 stone woman will still need a min of 1200 cals to survive.
Also, i would suggest that if your new to the tablets, stick to counting the fat. Then as your weight loss tapers off then add the cals too. I just did fat and found i was eating about 900 cals and wasnt losing much because i didnt realise how little i was eating in desperation to not have side effects! lol So when i spoke to the Dr, she said a min of 1000 was fine, but still wasnt losing enough, but since upping to 1200-1500 i have been doing well.