Trim the excess fat off the lamb and put the leg on 2 large sheets of baking paper. Make incisions all over with a small sharp knife and push in the garlic slices.
Mix 50ml olive oil with the lemon juice and drizzle over the lamb, then season with sea salt and rub with the remaining herbs and spices.
Cover with another piece of paper and fold the bottom pieces over the top piece to form a tight parcel. Secure with string. Marinate in the fridge overnight or for at least an hour. Remove from fridge and leave to come up to temperature while the oven heats to 150C/fan 130C/gas 3.
Put the lamb in a roasting tin with the end of the leg slightly elevated to stop any juices running out (just rest it on the edge of the tin). Bake for 3 hours.
Remove from the oven and rest for 30 minutes before unwrapping, carving and serving with the cooking juices.