Preheat the oven to 180oC, gas mark Grease and base line a 1 kg loaf tin.
Place 50g walnuts in a food processor and blitz to a coarse powder, chop a further 25g.
Whisk the butter and sugar in a large bowl until pale and fluffy. Whisk in the zest of 2 lemons and then whisk in the eggs, 1 at a time. Fold in the flour, baking powder and prepared walnuts to give a smooth batter. Spoon into the prepared tin and bake for 35-40 minutes until golden and a skewer comes out clean.
Just before the cake comes out of the oven, mix the juice of 1 lemon with granulated sugar. Chop the remaining 25g walnuts and stir into the syrup. Prick the cake with a skewer several times and drizzle over the lemon syrup. Allow to cool a little in the tin before removing then allow to cool completely.