1. Naje a slit down the back of each prawn and open out into butterflies.
2. In a bowl, mix together the remaining ingredients until well blended.
3. Add the prawn, turn to coat well then leave to marinate for 1 hour.
4. Preheat the grill for 5 minutes.
5. Put the prawns together with their marinate, to a shallow ovenproof dish (large enough to take them in one layer).
6. Grill for 10-15 minutes, turning and basting several times during the cooking.
7. The prawns are ready to serve.