Cut lamb into approx 500 gram pieces. Score 4-5 time longways with the grain of the lamb
Mix the marinade together in a bowl. Toss the lamb in the marinade, and leave covered in the fridge overnight.
Heat the BBq to a searing temperate and seal the lamb till brown on both sides – about 6-8 mins each side. (Can skip this step)
Place into a deep oven proof tray fat/skin side up with marinating juices,(just enough room for the lamb, but not overlapping). Cover with baking paper then foil.
Place in oven at 170c for approx 1.5 hours.
Check internal temp has reached at least 78c then take out out the oven and cool.
Pour cooking liquids into a pot or container and place into fridge to cool completely ,until a layer of fat has set on the top.
Place Lamb into fridge once internal temp has dropped to 20c.
Once lamb is cooled, carve into 6-8 reasonably thick pieces and place into an oven ready dish to re-heat.
Once the juices have cooled skim away 75% of the set fat. Pour off 20% of the juices and reserve. Measure the volume of the remains juices and fat.
Make a gravy with the 80% remaining by adding 50 grams of cornflour per litre of juices. Mix cornflour in 1 cup of water and add an extra 10 grams of cornflour. Heat and blend with stick blender until thickened and smooth.
With the lamb in oven trays add the reserved cooking juices to the lamb, ladle some gravy over the lamb, cover with paper and foil.
Re-heat the lamb at 200c for at 1 hour or until internal temp is 75c - 80c.
Serve with remainder of tray in a jug,.