Mix all the marinade ingredients into a paste. You can put all the ingredients into a blender and blitz to save time.
If you haven't asked your butcher to do so, cut slits into the lamb, this will allow the marinade to seep deeper into the lamb.
Then rub the marinade into your leg of lamb. Cover with foil and put in the fridge to marinate overnight.
When you are ready to cook the lamb, pour the water into the base of the tray. Then recover with the foil and try and make sure the foil is not touching the lamb, and that there is an air gap. This will stop the lamb burning where it meets the foil.
Then put the lamb into 140 C fan assisted oven. After an hour and a half, baste the lamb with the juices forming in the base of the tray. Re cover with the foil and put it back in the oven.
Turn the heat to 160 C fan assisted, and cook for an hour. Baste the lamb again after this hour.
Then put the lamb back in the oven and cook for another hour, or until you are happy the lamb has cooked to your liking. We like our lamb well done so we cook it well over three hours in total.
When the lamb has cooked, cover in foil and place on your worktop to rest. Let the lamb rest for at least 30 minutes.
Whilst the lamb is resting, pour the juices from the tray into a saucepan, and gently heat on the hob. There should be enough liquid in the pan for a gravy, if not, add some more water to the pan.
When the gravy in warm, stir in the cornflour and whisk into the gravy until there are no lumps.
Then add the cream and black pepper, mix through and simmer for a few minutes, until the gravy has thickened.
Pour into your gravy boat when ready to serve the lamb.