Dice the lamb into roughly bite-sized pieces, removing large pieces of fat and sinew as you go but don't over-trim - you want some fat left on to help add flavor as it cooks.
Place the lamb in a freezer bag or dish along with the rest of the ingredients - oil, lemon, yogurt, cumin, paprika, sumac and oregano. Mix all together well then seal or cover and refrigerate. Leave to marinate for at least two hours but overnight even better.
When ready to cook, warm up the grill/barbecue then thread the meat onto skewers. If using wooden skewers, make sure you soak in water in advance. While not traditional, I sometimes use some chunks of onion every few chunks of meat.
Cook over a hot grill a few minutes each side (exact time depends on grill heat and thickness of meat) until meat is just cooked through. Serve with eg bread and salads.
