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Preheat the oven to Gas 4, 180°C, fan 160°C.
Grease a (20cm) round, deep loose-based tin and base line with baking parchment.
Place the butter sugar and lemon zest in a large bowl. Use an electric whisk to beat the butter and sugar together until they are pale and fluffy. Gradually add the eggs, whisking well between additions and adding 2 tbsp of the flour with the last egg — this will prevent curdling.
Sift over the remaining flour, then gently fold in with a metal spoon along with 1 tbsp hot water. Spoon into the prepared tin, level the surface and bake for 50-60mins until it is shrinking away from the sides of the tin. A fine skewer inserted in the centre should come out clean. Cool in the tin for 5 mins.
Squeeze the lemon juice, then sieve to remove the bits. Stir the granulated sugar into the lemon juice. Use the fine skewer to prick the cake all over, pour over the syrup — it should sink in but leave a crunchy crust. Leave to cool completely.
Serve in wedges.