Heat the oven to 180°C.
To prepare the stuffing:
Slice half the chestnuts finely and place them in a bowl with the sausage meat, shallots, garlic, parsley, sage, seasoning and egg. Mix well — this is best done using your hands.
To prepare the turkey:
Remove the giblets from the turkey and keep aside, then season the inside of the turkey and stuff with the freshly made mixture. Generously smear the outside of the turkey with butter.
Grease a heavy-based, deep roasting tin with a little of the remaining butter and add the giblets, celery and the whole carrots and onions. Pour over half the wine and half the chicken stock, and place the turkey on top. Cover generously with tin foil and cook in the middle of the oven for 1 1/2 hours.
Remove the turkey from the oven and reduce the heat to 150°C/gas 2. Add the whole chestnuts and the remaining wine and stock, smear the turkey again with what remains of the butter, re-cover and return to the oven for a further 1 - 1 1/2 hours.
For the final 10 minutes of cooking, remove the tin foil and increase the heat to 180°C/gas 4 to give the turkey a golden skin. Remove from the oven, lift the turkey onto a carving plate and leave to rest in a warm place for 10-15 minutes - this allows the meat to relax and become tender.
Discard the giblets. Carefully lift out the vegetables and chestnuts from the roasting tin and keep warm.
Place the roasting tin directly on the heat and, using a flour dredger, shake in about 1 tablespoon of flour. Whisk vigorously and bring to the boil to make a coating gravy - adjust with more liquid (stock or water) or a little more flour to get the right consistency. Pass through a fine sieve, then check the seasoning.
Serve the turkey with the chestnuts, vegetables and sauce.