Preheat the oven to 180 C / Gas 4. Grease a 2 lb loaf tin, and line with baking parchment.
Combine the flour, sugar, bicarb and poppy seeds in a mixing bowl. In a separate bowl, combine the yoghurt, butter, oil, eggs and vanilla. Tip the wet mixture into the dry mixture, and gently combine until just moistened. Spoon into the prepared loaf tin.
Bake for 45 minutes to 1 hour, or until a skewer inserted in the centre comes out clean. Cool before slicing.
To give a lemon flavour to the cake, add 1 dessertspoon of lemon zest to the dry ingredients. You can also top the cake with a lemon or orange drizzle by combining icing sugar with lemon or orange juice till your desired consistency is reached.