Preheat the oven to 400°F/200°C/Gas 6. Wash the giblets in cold water, place in a large pan with 1.5 litres of cold water. Bring to the boil and remove any scum off the top of the water with a slotted spoon. Then add the rest of the ingredients, cover and simmer for 40 minutes.
Uncover the pan and simmer for a further 20-30 minutes or until the stock has reduced by half. Remove from the heat and strain into a bowl, leave to cool, cover and place in the fridge or use immediately.
To make the gravy, bring the stock back to the boil and remove from the heat. When the turkey is cooked pour off the fat from the roasting juices, place the tin on the hob, whisk in the flour and place over a low heat. Cook the flour mixture making sure you have scrapped up all the meaty sticky bits on the bottom of the tin and stir constantly.
After 2 minutes, remove from the heat and whisk in the turkey giblet stock and redcurrant jelly, bring slowly to the boil. Add the sherry and cook for a further 3 minutes, stirring constantly. Season to taste, add a little more sherry if you want and then strain into a warm jug and serve. Any leftover turkey sherry gravy can be frozen and the giblet stock can be made 2 days before required.