In a bowl, combine the yeast, sugar, 2 tablespoons of the warm water and 2 tablespoons of flour. Leave in a warm place for 15 to 20 minutes.
In a mixing bowl, combine the remainder of the flour with the remainder of the warm water, olive oil, salt and yeast mix and bring together as a dough. Turn out onto a lightly floured work surface and knead for 10 minutes until elastic. Place in a lightly oiled bowl in a warm place and cover with a tea towel, allow to rest for 1 hour.
Preheat the oven to 230 C / Gas 8. Lightly grease two (30cm) pizza trays.
Turn the dough out onto a lightly floured work surface and divide in two (or 4 parts if you are making mini-pizzas). Shape dough into a circle, carefully pulling on the edges, and roll out with a rolling pin. Place onto the prepared pizza trays.
Place the tomatoes in a bowl and cover with boiling water to scald for about 30 seconds. Remove from the water, peel off and discard the skin then finely chop the tomato.
Place tomatoes, thickly sliced leeks and basil on to the pizza base. Sprinkle with oregano then lay slices of mozzarella cheese on top.
Bake in the preheated oven for 15 minutes until the pizza dough is risen and golden brown and the cheese is melted. Remove from the oven, slice and serve.