Preheat the oven to 180°C (fan oven 160°C), gas mark 4. Line a 900g (2lb) loaf tin or a 20cm (8in) round cake tine with non-stick baking parchment.
Place all the ingredients except the conserve into a mixing bowl and, using a hand-held electric whisk, blend the ingredients together until smooth. Alternatively, beat with a wooden spoon for 2 mins.
Spoon into the prepared tin and level with the back of a spoon. Place the wild blueberry conserve into a paper piping bag and snip off the end. Pipe a swirl of the conserve on the top of the cake.
Bake on the centre shelf in the preheated oven for about 40-50 mins until a skewer pushed into the centre comes out clean. Sprinkle with a little extra caster sugar. Cool in the tin for 10 mins, then turn out and leave to cool completely on a wire rack.
The cooled cake will freeze for up to 1 month. To use, thaw overnight at room temperature.
If you don't want to pipe the conserve onto the cake mixture, just drop it from the end of a teaspoon.