Sift the flour into a large bowl. Add the eggs & stir until combined. Add the milk & beat into a batter. Cover & leave to stand for 30 minutes.
Sprinkle the peppercorns onto a clean chopping board & press the rump in the peppercorn. Place the whole rump on a baking sheet & drizzle with 1 tbsp oil.
Roast for 15 minutes for medium rare. Remove & cover in foil to rest.
Place the muffin tray on a baking tray & pour 1/4 tsp of oil into each of the muffin holes. Place in the oven for 5 minutes. Remove from the oven & immediately pour approx 1 tsp of the batter into each hole. Bake for 12 minute until puffed & golden brown.
Meanwhile slice the beef very thinly. Remove the Yorkshire puddings from the tins. Top with the steak & spoon over the horseradish sauce and garnish with parsley.