As mentioned above, it's all to do with body fat %age, not weight. Sub 10% and you'll have visible abs, roughly 15% is where you look neither fat nor thin. For a 6 ft male, I'd say that's roughly 12 stone ( I'm 6 ft 2, and when I weighed 12.5 stone I had a BF of 12.5%)
Oh and @ Oliver - you do not have 13 stone of muscle if you weigh 17 stone... your skeleton will weigh roughly 6.8 stone, and you've got 3.06 stone of fat, so even if the rest of your body is muscle (which it's not), you can't have more than 7 stone of muscle.