Grease a 23cm cake tin and heat oven to 180C/160C fan/gas 4. Put the lemons in a saucepan and cover with cold water. Bring to the boil, drain, pour over more cold water, then simmer for 1 hr until the lemons are really soft, topping up with more water if you need to. Drain the lemons well. When cool, halve them, remove the pips and blitz in a food processor to a purée.
Add all the other cake ingredients and carry on blitzing until you have a loose batter. Scrape the batter into the cake tin and bake for 40-50 mins until golden and starting to shrink away from the sides. While the cake bakes, make the drizzle by mixing the sugar with the lemon juice. When the cake is ready, remove from the oven and leave to cool a little. While still warm, pour over the drizzle and leave to cool completely. Serve with mascarpone.