In a large saucepan, sauté the garlic in the olive oil for 1 min, then add the tomatoes and bring to the boil. Pour in the vinegar and sprinkle in the sugar and add the cinnamon stick, if using, then bring to a simmer just below boiling point. Hold the heat at this point, stirring occasionally, for 25 mins. Allow to cool and store in clean, sterilised bottles. It will keep, stored in the fridge for 1 week.
Meanwhile, place the potatoes in a saucepan of cold water and bring to the boil. Cook for 10 mins, then drain and cool. In a large bowl, mash the cooked potatoes, then mix in the onion, egg and plain flour.
In a large frying pan, heat the rapeseed oil until sizzling. Add the potato mixture and flatten out into a large, thick potato cake. Cook for 3-4 mins on each side, until golden. Turn out onto a board and cut out the Christmas tree chips with a tree-shaped cutter. Serve, dressed with Sarson’s malt vinegar and the tomato sauce.