A stone is a metric measure of weight. Gah!
Erm, just diet I suppose. And exercise regularly. You can try cutting out certain products, i.e. cut out, use less, or use an alternative to butter - go from whole or semi - skimmed milk to skimmed milk.
Try not to snack, have breakfast every day, and cut out the fizzy and the chocolate!
Hope this helps, good luck!
Edit: Try going jogging once in a while, or take up a new sport such as badminton (An indoor sport you can play whatever the weather!) Maybe avoid driving at times where you can simply walk, or even cycle. Unless it's tipping it down with rain of course...