Once the turkey has cooked and is resting on a board, carefully pour the juices from the roasting tin into a large heatproof jug. Leave to settle, then pour all of the fat into a separate jug. Put 2 tbsp fat in a saucepan and carefully dispose of the rest. If you have lots of pan juices left after skimming the fat off, substitute this for some of the chicken stock. Alternatively, put the goose fat or butter in a saucepan.
Put the saucepan with the fat in on the heat and add the giblets. Cook for 10 mins or until golden, then add the flour. Cook for 2-3 mins, then add the white wine and stir well. Pour in the chicken stock, then add the thyme and bay. Bring to the boil, then reduce the heat and simmer for 25-30 mins or until reduced by half, then stir in the chestnut purée.
Strain the gravy into a jug, discard the herbs and giblets, season and serve with turkey.