Heat oven to 170c/150c fan/gas 31?2. Grease and line a 2lb loaf tin with baking parchment. Put the butter, caster sugar and lemon zest in a large bowl, and beat with an electric hand whisk until pale and fluffy. Crack in 1 egg, whisk into the butter mixture until well combined, then add 1 heaped tbsp flour and mix again. Continue with the remaining eggs and flour, finishing with any remaining flour and mixing until just combined. Finally, add the yogurt and lemon curd, and fold together with a spatula until smooth. Scrape the mixture into the tin and bake in the centre of the oven for 45-50 mins until golden and risen – a skewer inserted into the centre of the cake should come out clean.