For the candied peel, cut the lemon into 8 wedges, then cut out the flesh, leaving a 5mm thickness of peel and pith. Cut each wedge into 4 strips.
Pour 300ml/10fl oz water into a small pan, add the granulated sugar and heat, stirring until the sugar has dissolved. Add the lemon zest and simmer uncovered for 30-45 minutes, until the lemon peel is soft and translucent.
Drain in a wire sieve and leave to cool. Spread the peel out on a baking sheet lined with parchment and sprinkle with caster sugar. Place in the oven on the lowest setting (approximately 110C/90C Fan/Gas ¼) for 1 hour until dry.
For the cake, preheat the oven to 180C/160C Fan/Gas 4. Grease a 18cm/7in deep round cake tin and line the base and sides with baking parchment.
Put the butter, sugar, flour, ground almonds, eggs and lemon zest into a large bowl. Using an electric mixer, beat for 1 minute to mix thoroughly. Turn into the prepared tin and level the surface.
Bake for 35 minutes. Decorate with 3 pieces of candied peel and bake for a further 20-25 minutes, until a skewer inserted into the centre, comes out clean. Leave in the tin for 15 minutes to cool before turning out onto a wire rack to cool completely.