Cut the celery into thick slices, top and tail the carrots and scrape and cut into thick slices, peel and quarter the onion, and remove some of the skin off the garlic and trim the base. Add all of these with the game scraps to a shallow pan, pour over the olive oil and put in an oven at 425°F/210°C/Reg7.
Cook the meat and vegetables for about 20 minutes, turn them over and cook for another 20 minutes. Put the meat and vegetables along with any liquid into a large saucepan. Trim and peel the leeks, cut into thick slices and add to the saucepan. Tie the thyme in some string and add to the saucepan along with a good amount of black pepper. Pour over water to just cover and simmer covered for about two hours.
When ready spoon off the meat and vegetables, and use a sieve to drain the liquid into a saucepan. Put the saucepan over a high heat and reduce by half to deliver around 1 pint/½ litre. Allow to cool a bit and season well with salt, and more black pepper if required.
Put the stock covered into the freezer and leave over night preferably. Take out the freezer and spoon off any fat from the top. Use the stock as required.