Heat the oil in a large saucepan. Add in the bacon and sauté over a high heat until crispy. Remove to a plate. Add a knob of butter to the saucepan, reduce to a medium heat then add in the leeks and garlic. Fry gently for 5 minutes until softened, but not coloured. Add the potatoes and cook for a further minute. Pour in the stock and lemon rind, cover and simmer for 12-15 minutes or until the potatoes are tender. With a slotted spoon, remove half the potatoes and leeks from the stock and set aside.
Transfer the remaining potatoes, leeks, stock and milk into a blender or food processor and whizz until smooth. Pour back into the saucepan, add the sweetcorn, bacon, fish and the potatoes and leeks you have set aside. Cover and gently heat for 3-4 minutes until the fish is just cooked through - don't boil. Stir in chives and cream, if using, then season to taste.