For the marinade, blend all of the marinade ingredients except the olive oil to a smooth paste in a food processor (Alternatively, grind to a paste in a pestle and mortar.) Gradually add a drizzle of olive oil to loosen the marinade mixture as necessary.
For the lamb, transfer the marinade to a resealable freezer bag and add the lamb rumps. Seal the bag, removing as much air as possible as you do so, then massage the mixture into the meat and set aside in the fridge to marinate for between 2 hours and 24 hours.
When the meat has marinated, either preheat the oven to 190C/375F/Gas 5, or prepare a barbecue.
Remove the marinated lamb rumps from the freezer bag and shake off any excess marinade. Place the marinated lamb rumps into a roasting tray and roast in the oven for 20-25 minutes (for medium, depending on the size of the rumps), or until cooked to your liking. Alternatively, cook the lamb rumps on the hot barbecue grill.