The liquid content of yoghurt varies from brand to brand, but what you need for this recipe is the thickest you can find. The key is to drain it until it has a very thick, spreadable consistency. To quicken the process, divide the yoghurt between two muslin-lined sieves or colanders, suspended above large bowls. Serve the cake with extra strawberries and orange segments if you like - blood oranges, when in season, work well. You'll need to begin this recipe a day ahead.