Heat oven to 180C/160C fan/gas 4. Lightly oil a 20cm round x 5cm deep cake tin and line the base with baking parchment. For the cake, put the flour, baking powder, ground almonds and polenta in a large mixing bowl. Stir in the lemon zest and sugar, then make a dip in the centre. Beat the eggs in a bowl, then stir in the yogurt. Tip this mixture along with the oil into the dip (see step-by-step number 1), then briefly and gently stir with a large metal spoon so everything is just combined, without overmixing.
Spoon the mixture into the tin and level the top (step 2). Bake for 40 mins or until a skewer inserted into the centre of the cake comes out clean. Cover loosely with foil for the final 5-10 mins if it starts to brown too quickly.
While the cake cooks, make the lemon syrup. Tip the caster sugar into a small saucepan with the lemon juice and 75ml water. Heat over a medium heat, stirring occasionally, until the sugar has dissolved. Raise the heat, boil for 4 mins until slightly reduced and syrupy, then remove from the heat.
Remove the cake from the oven and let it cool briefly in the tin. While it is still warm, turn it out of the tin, peel off the lining paper and sit the cake on a wire rack set over a baking tray or similar. Use a skewer to make lots of small holes all over the top of the cake (step 3). Slowly spoon over half the lemon syrup (step 4) and let it soak in. Spoon over the rest in the same way, brushing the edges and sides of the cake too with the last of the syrup.