I disagree.
Yes an ab roller or sit up regime will tone and strenghten the stomach muscles but that is of little use if you do not lose the fat.
Remember Geoff Capes? Well I can assure you he had a six pack and stomach muscles stronger than you and I put together. But he was also a little chubby which hid the muscles.
In fact an ab roller may make your waist larger as abdoms tend to be a protuding muscle. Hence they will push the gut out more.
I too have a 36 inch waist but have a larger chest than you which gives a better proportion.
It will easier to build the chest than lose the gut, trust me.
But all that said, if you are naturally a big lad, 15.5 stones seems a healthy weight. Forget all those bloody charts etc. According to them Frank Bruno is clinically obese. Do you want to tell him that?
If you really want to lose a few inches just cut back on a few snacks and do a bit more walking. As a general rule of thumb, the last weight on is the first weight off. Although you can not "spot reduce", the formation of brown fat cells are easier to shift than established ones. And if you are like me, the older we get, the fat tend to be on the belly.