About 10 mins before baking, heat the oven to 220C/200C fan/gas 9 and mix 2 tsp milk with the Marmite, if using. Line a baking tray with baking parchment. Scatter half the remaining parmesan and all the grated cheddar over the tray, then sit the scones on top. Brush the tops with the milk and Marmite mixture (or just milk, if you prefer), then scatter over the remaining parmesan and the thyme sprigs. Bake for 20-25 mins until the scones are deep golden brown and puffed up, and the cheese on the tray is crisp. Cool on the tray for 10 mins, then serve warm as is or slathered with butter.
