Cut one lemon into six slices and the other lemon in half. Put the lemon slices in the base of a shallow sided casserole dish or pan, then put the chicken on top. Place the two lemon halves into the cavity of the chicken and tie the legs together with cooking string. Place the extra thyme sprigs around the chicken.
In a small bowl, mix together butter, herbs, garlic, pepper and salt. Carefully push your fingers between the skin and the breast meat of the chicken to create a pocket, do the same with the tops of the legs. Spoon ¾ of the herb butter under the skin and spread it out evenly. Smear the remaining butter over the outside of the bird.