1) Make the marinade by mixing together the buttermilk, yoghurt, mustard, hot sauce, tarragon, paprika and salt in a small bowl. Put the chicken in a heavy resealable plastic bag or a baking pan large enough to hold the pieces in one layer, and pour the buttermilk over it. Seal the bag or cover the pan with clingfilm and marinate in the refrigerator for about 8 hours.
2) Preheat the oven to 230C/Gas 8.
3) Grease a baking pan large enough to hold the chicken pieces in one layer.
4) Place the flour and oatmeal in a brown paper bag. One piece at a time, place the chicken in the bag and shake until coated. Place the pieces skin side down in the prepared baking dish. Continue until all the pieces are covered in the flour mixture. Bake for 40-45 minutes, turning once, until the chicken is golden brown.