Hmm,
A stone is 14 pounds...and I thought the American system was screwed up :D:D:!
So, that's 56 pounds in 6 months or just slightly more than 2 pounds a week.

I'd say yes, IF on a very strict calorie reduction diet. With weight loss this high, diet is far more important than type of exercise. Eat very little (1200 calories TOPS) and walk 5 miles a day. But be warned, this kind of weight loss is at the very upper limit of doability and it's probably wise to be under a professional's supervision to do it.

I think a pound a week is wiser...like 4 stone a YEAR!