I googled Gambas Pil Pil and discovered that this seemed to be shelled prawns 'boiled' in hot garlic and chilli infused oil. So, instead, I just marinaded the prawns in their shell, in their bag with a 1tsp olive oil, one crushed garlic clove, a pinch of chilli flakes, and a pinch of rock salt. for most of the afternoon.
When ready to barbecue, thread the prawns onto two skewers so you can turn the whole lot over easily during cooking (NB use a cloth to pick up the skewers as they get bl**dy hot as I discovered). Cook for about 5 minutes turning a few times until browned on the outside. Leave to cool a little before peeling!
Oh and don't forget to keep the heads, shells, etc. They make a great Soupe de Poisson.