I personally think 900 cals is a little on the low side for 15st 11. For my weight of 10stone (whilst I am still dieting) I think that's fine - others will disagree and say it's too low for me, but I don't have 900 every day - maybe only 2-3 days per week.
If you calculate your BMR (without the harrison benedict formula) it's a base calorie amount of 1805. Mine is 1400 which shows I should definitely be consuming less calories as we both diet. I would never go below 800/900 per day for my size so I would think 1200minimum would be much better for you. Mainly because you're much less likely to fall off the wagon if you allow yourself some more calories. Also the reason you need more calories is to sustain the energy required for the weight each of us are - if you skimp you could end up with no energy!
It's all very personal view when it comes to calories / weightloss. I have around 5 consultant / surgeon friends and they all say that the whole starvation mode (where you don't eat enough and start storing fat) only really applies if you were 100% disciplined every single day of your life, never having a day of naughty food and eating insanely low calories. That's why body builders eat like saints for 6 days a week then have a cheat day to shock their body. But their good days are not like ours!! hahah And even then you would still lose weight just maybe not quite as fast. At the end of the day it's a simple mathematical equation - if you eat fewer calories than your body needs to stay the same you will lose weight - that's a fact!
But this is only my point of view - others will argue that eating 1800-1900 a day is fine - but given the recommended amount for an average adult (including men!) is 2000 that seems a little high for a diet! No offence to anyone intended xx