For the pancakes: put the potatoes in a pan and cover with cold water. Add a pinch of salt, cover and bring to the boil. Cook for 15-20 minutes until cooked.
Drain the potatoes well. Mash half of them and coarsely grate the remainder.
Put the leeks in a pan and cover with plenty of lightly salted water. Cover and cook for 3-5 minutes until tender. Drain well then press with kitchen paper to extract as much moisture as possible.
Sift the dry ingredients into a bowl, and mix in the mashed and grated potatoes and the leeks.
Stir in enough buttermilk to make a thick batter.
Heat the butter in a non-stick frying pan. Spoon in some of the batter and cook for about 2 minutes or until the top of the pancake is silky and bubbly. Turn over and cook for a few more minutes until golden and puffy. Repeat with the remaining batter. Keep the pancakes warm.
For the topping: heat the oil in a separate pan and cook the bacon until golden and crisp. Add the butter, sage and lemon juice.
To serve: put the pancakes on serving plates and spoon the bacon mixture over the top.