Now add the self raising flour and a pinch of salt plus the rind and juice of one lemon. Combine until a thick batter is created. Add the blueberries and gently fold them in. The cake batter will appear to be very thick, my reason for doing this is because as the blueberries cook they'll release juice and make the batter wetter. So don't worry about it's appearance at this stage. It should look more like a wet bread dough.

Grease your cake tin with butter then spoon in the batter. Smooth it all up the the edges and then dollop over the lemon curd and work it through the batter in swirls using a knife or spatula, once you've done that smooth the batter over again so it's nice and neat.