Heat the oil in a large non-stick saucepan over a medium heat. Add the onion, carrot and pepper and fry for 4-5 minutes, stirring regularly, until softened. Add the garlic and fry for 1-2 minutes, stirring regularly.
Add the remaining tomato sauce ingredients and 100ml/3½fl oz water, stir well to combine. Season with salt and pepper. Bring to the boil, then simmer for 18-20 minutes, stirring regularly, until the vegetables are very soft and most of the liquid has evaporated.
Remove the bay leaf, blend to a very smooth purée using a hand-held blender.
Preheat the grill to its hottest setting.
Place the pitta breads onto baking trays and spread some of the tomato sauce over each. Sprinkle over the ham and grated mozzarella. (At this point, the pizzas can be frozen on baking trays, then packed into freezer bags, ready for another day.)
Grill the pizzas for 2-3 minutes, or until the bread is piping hot and the cheese has melted and turned golden-brown. Serve immediately.
To cook from frozen, preheat the grill to a medium setting, then grill for 5-6 minutes, until piping hot throughout.