For the pears: Pour 750ml water and the caster sugar into a medium pan. Then, using a sharp knife cut off five strips of lemon zest and add to the pan. Slightly crush the cardamom pods, (enough to crack them but not release the seeds) and also add to the pan. Squeeze the lemon juice into a medium bowl. Cut the base off of the pears to give them a flat bottom, then peel leaving the stalk intact. Then core using an apple corer and spoon to get the seeds out. Place them into the bowl of lemon juice as you work, to prevent them from browning. Meanwhile bring the sugar mixture to a gentle simmer over a medium heat, then add the pears and lemon juice to the pan and poach for 15-20 minutes until tender. Remove the pan from the heat and place the pears on a cooling rack set over a baking tray to drain. Then set aside while you prepare the cake.
For the cake: Grease and line a 2lb loaf tin and preheat the oven to 170C/150C Fan/Gas Mark 3. In a large bowl, beat the butter and sugar together using an electric whisk. Then add the eggs, one by one, mixing well between each addition. Mix the dry ingredients together in a separate bowl, and then add to the wet batter in two batches. Mixing only until just combined. Finally, stir in the milk until combined.
To assemble: Place the pears upright in the prepared tin, then spoon the batter around the pears. Bake for 1hr-1hr 5mins, until a skewer comes out almost clean. Let cool for 10 minutes in the tin, and then remove and place onto a cooling rack to cool completely.