Beer belly is only a common description. You don't need alcohol to gain fat around the middle.
Lunch sounds very light. Salad and maybe chicken for what I assume is your main meal, sounds very light also. You may have forced the body to conserve, although the rest of your post suggests it may not be that. Telling us the total amount of calories you consume each day would help.
Cut down on the peanuts and limit the alcohol you do have. Without knowing quantities it's difficult to judge the actual effect.
12 stone is probably a little overweight (you don't give your height) but well below obesity. And the body changes as one ages, fat tends to go to the middle.
I'm wondering if you simply have an incorrect body image and are carrying the same as everyone else, or possibly even a little less.