Have you actually put on a stone, or have you simply regained the weight you lost when you had undiagnosed type 1 diabetes? Without insulin your body can't get any energy from food, even though you are eating your body thinks you are starving and it starts breaking down your fat and muscle tissue. With undiagnosed type 1 you are starving to death.
When you start taking insulin your body is desperate to put back on the weight lost when you were so poorly, so you will quickly put the weight back on to go back to your healthy weight.
So, consider whether when you are thinking about weight gain where your starting weight is. Don't measure yourself against a weight you were because you were so poorly you were basically starving to death.
If you think you are actually putting on weight, a type 1 diabetic looses weight the same as anyone else: diet & exercise. You need to be able to carb count to adjust insulin doses when changing your diet, you need to be able to adjust insulin doses to take account of exercise.