Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and line a baking tray with grease-proof paper.
Sieve the flour into a bowl and create a well in the middle. Mix the yeast with 5 tbsp milk and 1 tsp sugar and pour into the well. Loosely mix in a little flour then cover and leave to rise in a warm place for around 30 minutes.
Mash the potatoes and mix with the margarine, the remaining milk and sugar, the lemon zest and a pinch of salt.
Add the potato mixture to the flour mixture and knead well. Leave to rise for another 45 minutes.
Roll out the dough and place it on the prepared baking tray. Pierce with a fork, sprinkle with the slivered almonds and bake for 30-40 minutes until golden. Remove from the oven, leave to cool and serve in slices.