Heat oven to 180C/160C fan/gas 4. Lightly oil and line a 1kg loaf tin with a strip of baking parchment. Finely grate the courgettes, then squeeze out as much liquid as you can with your hands (too much will make the cake soggy).
Stir the courgettes with the sugar, sunflower oil, eggs, orange zest, vanilla and sultanas, then fold in the flour and baking powder until they disappear, but don’t overmix.
Scrape the mixture into the tin and bake for 50 mins until a skewer inserted into the cake comes out clean. Remove from the tin and cool on a wire rack.
Meanwhile, beat the cream cheese with the icing sugar (don’t overbeat as this can make the mixture too soft), then chill. Spread over the cooled cake, scatter with the zest and chill until ready to serve. Will keep for a week in the fridge.