I agree with jwilso, in that you want to be sure you know why (and if) it is important to isolate the stretch to the hamstring muscle belly.

If there is a sound reason to isolate the stretch to the muscle belly, the bending of the knee you described often works, but only if you don't lose the position of the pelvis. A lot of people posteriorly tilt the pelvis when stretching the hamstring, losing the tension on the muscle. To know if you are doing this, perform the following test:
Stand upright in a neutral position. Place one hand, with fingers stretched out, in the low of your back.
This hand is there to detect movement of the lumbar spine during the upcoming hamstring stretch - there should be zero movement.
Place your foot onto a stable object, with your knee slightly bent.
Keeping your low back's normal curve (still feeling for movement with that hand), bend forward at the hips, performing your typical hamstring stretch.
If your hand feels your low back flatten, then you are losing your pelvic tilt.

To see correct pelvic position during a hamstring stretch, compare the pictures on this page (good maintenance of lumbar curve) to the picture on this page (pelvis tilts backwards, losing the lumbar curve).

Of course, if you want to isolate the muscle belly because of symptoms there, also be advised that this may not be where the source of the problem is.