Mix the ketchup with roughly equal amount of water and set aside. In a large cup, combine the paprika powder, basil, thyme, rosemary, garlic, and salt. Stir in the ketchup and water mixture, then slowly add the oil while stirring until well combined.
Place the pork fillets on cling film in a baking dish. Apply the marinade to all sides and massage lightly. Cover with cling film and refrigerate overnight.
Prepare a kettle grill for indirect grilling. Sear the pork fillets on all sides for 2-3 minutes over medium-high direct heat. Move to the indirect zone in the middle of the grill, close the lid, and grill at 180°C (356°F) for 25-30 minutes until the internal temperature reaches 63°C (145°F).