Heat the oven to 200C/fan 180C/gas 6 and put 8 muffin cases in a muffin tray. Mix the flour, baking powder and sugar in a large bowl, add the melted butter, soured cream, eggs and lemon zest and beat just long enough to give a thick, smooth mixture - don’t over-beat. Carefully stir in the fruit and finally the sparkling water, until just evenly combined.
Divide the mixture between the muffin cases - they should be nice and full. Sprinkle generously with the sugar and bake for 25-30 mins or until risen and golden. The muffins are best served just warm. Will keep fresh in an airtight container in a cool place for 1-2 days.