Start this recipe by seasoning the chicken pieces and toss with the flour. Heat the oil in a large saucepan and brown the chicken pieces in 2 batches. Remove from the pan, add in the butter, leek, carrots and celery and cook together for 4-5 minutes. Pop the chicken back into the pan, along with the chicken stock and the fresh herbs, bay leaf and hot stock. Bring to the boil, lower the heat and cook for 35-40 minutes. Add in the peas and cream, remove the herbs and cook for a further 5 minutes. For the mashed potatoes, heat the remaining 100ml single cream and 400ml milk into a saucepan, with 1 knob butter and a good pinch of salt. When warm, add in the contents of the mashed potato sachet, stir together well and continue cooking until smooth and creamy. Pipe the mashed potato onto 4 serving plates and spoon over the hot chicken stew.