Combine the marinade ingredients in a bowl and mix well. Put the pork and the marinade in a resealable plastic bag. Seal the bag and shake around to coat the meat well. Refrigerate overnight.

Preheat the oven to 180 degree celsius. Line baking tray with kitchen parchment paper and place a roasting rack on it.

Remove the pork from the bag, reserving marinade, place on the rack and roast for about 40 minutes. As it roasts, use a brush to baste it 3 or 4 times with reserved marinate. Save some marinate to make the sauce later.

Discard the green part of the leek. Trim and thinly slice the white part on the diagonal and soak in the bowl of cold water for about 3 minutes. Drain.

Pass the reserved marinate through the sieve or tea straining into a small pan. Simmer for a low heat while stirring with a wooden spatula for about 5 minutes or until thick.