Dissolve the sugar in 150ml/¼ pint of the lukewarm water in a small bowl and stir in the dried yeast. Sit the bowl for a few minutes in a warm place to allow the yeast to start to work. After about 4-5 minutes it will have a creamy, slightly frothy appearance.
Place the rice flour, potato flour, tapioca flour, dried milk, gluten-free baking powder, xanthan gum and salt into the bowl of a food mixer. Using the paddle attachment, thoroughly mix the dry ingredients together. In a small bowl, whisk together the sunflower oil and egg and stir into the dry ingredients on a low speed.
When the yeast mixture is ready, stir and pour with the remaining lukewarm water into the mixer bowl. Using the K-blade attachment on a low speed, mix the liquid through the contents of the bowl. Continue to mix for 3-4 minutes, until a smooth dough is produced.
Transfer the dough to a rice-floured work surface. The dough will be slightly sticky, so it helps if you put a little rice flour on your hands too. Divide the dough into four equal parts, each weighing about 150g/5oz. Place a sheet of parchment paper on a baking tray. Transfer one piece of the dough to the baking tray and, using the 'heel' of your hand, flatten it to form a circle measuring 20cm/8in in diameter. Repeat with the remaining three pieces of dough.
Cover the dough circles with a clean tea towel and allow them to rise in a warm place for about 15 minutes.
Preheat the oven to 200C/400F/Gas 6.
Transfer the baking sheets to the oven and bake for 8-10 minutes before removing and adding the toppings of your choice. Return the pizzas, with the toppings to the oven and bake for a further 10-15 minutes until the bases are crisp and the toppings are bubbly and golden.
Serve immediately.