Warm 3 tbsp olive oil with the rosemary in a pan over a low heat for 5 minutes. Take off the heat and leave to infuse for 10 minutes, then remove the rosemary and reserve. Meanwhile, barbecue or grill the smoked back bacon until crisp. Chop finely.
Place the remaining oil and the popping corn on two squares of foil. Fold into loose parcels, sealing the edges. Give them a shake, then put on the coolest part of the barbecue for 5-10 minutes until the popping stops.
Tip the popcorn into a bowl and mix in the bacon and rosemary oil. Sprinkle with chilli flakes and garnish with the rosemary sprigs.