If your weights have not dropped then you have not lost muscle tissue and have just lost BF, you must be a lucky one
I wouldn't be too concerned about your weight, it is definately more about appearance. When my BF gets down to a certain point I tend to rely on appeance more than what the scales say.
If you are dropping in bodyweight and not using steroids then a safe amount to be losing a week is 1lb as more than this is muscle tissue, with steroids you can afford to drop more as muscle tissue is retained easier.
Also do not have a calorie deficit of more than 700 calories a day, even though you may have dropped calories by 400 you may be burning an extra 400 throgh extra cardio. A safe amount to drop is 500 calories
I would say keep doing what you are doing it sounds if its going great. If I could keep at 16.5 stone and cut up I'd be happy but already dropped 7lbs.
J