Preheat oven to 180 degrees C. Line a 20cm cake tin.
Cream the butter and caster sugar until pale. Beat in the eggs one at a time.
Fold in the zest and flour, reserving the juice. Fold in the yoghurt. Scrape batter into the cake tin and bake 30-40 minutes until a skewer comes out clean.
Just before the cake is done heat the water, sugar and reserved lemon juice in a small saucepan and simmer 5 minutes. When the cake is done, remove from the oven and prick all over with a skewer. Drizzle the lemon syrup over and let cool.