How long would this take to loose a stone?

I thought you meant a kidney stone at first. I would say exercise (especially at your age) is much more effective than diet for weight loss. The biking is therefore good. And you're not too heavy either. It's hard to tell how long it will take to lose weight, it varies from person to person, but expect to at least see some decrease in the first 2 weeks if it is working at all.