The first week is most often the biggest loss for people. You're losing mostly water weight, then a few lbs of fat. Then it slows right down as you're only losing fat.
As for whether 2 stone in 2 months is realistic - I think it's a tough order. 1.5 stone might be more realistic but it will depend on what your bmi is and how rigidly you stick to the diet, whether you've just come from another diet, whether you find out anything stalls you, etc etc.