To make the marinade, put the cumin, coriander and fennel seeds in a pestle and mortar and pound to a course powder. You can use 1 1/2tsp each of ground cumin and coriander if you prefer, but the flavour won't be quite as good as freshly ground spices. Add the thyme leaves and crush them into the spices for a few seconds.
Tip the spices and thyme into a large non-metallic bowl and stir in the lemon zest and juice, garlic, oil, salt and lots of black pepper.
Trim as much fat as possible off the lamb, then cut the meat into rough 3cm chunks - you should have about 40 chunks. Add the lamb to the marinade and toss until well coated. Cover with cling film and chill in the fridge for 30 minutes before cooking.
While the lamb is marinating, prepare the vegetables for the kebabs. Deseed the peppers and cut them into 3cm chunks. Cut each onion into 8 wedges with the root intact.
Remove the lamb from the fridge and thread the meat on to 8 metal skewers - these should be about 25cm long - alternating with the pieces of pepper and onion. Season with a little more salt and pepper.
Cook over a hot barbecue or under a preheated hot grill (close to the element) for 6-8 minutes. Turn once or twice until the lamb and vegetables are lightly charred - the meat should be pink in the middle. Mix the yoghurt, garlic, and chopped mint or mint sauce together in a small bowl to make the yoghurt sauce.
Warm the pitta bread on the barbecue, in the toaster or under a grill and carefully cut down one side with a sharp knife. Pull the bread open and stuff with shredded lettuce leaves, grated carrot, tomatoes, and cucumber. Slide the meat and vegetables off the skewers and into the pittas using a fork. Drizzle a little of the minty yoghurt sauce and serve with some chilli sauce and lemon wedges too if you fancy.