Prepare the BBQ for direct cooking by spreading the coals in an even layer. Heat the oven to 200C/fan 180C/gas 6.
Put the gooseberries in a roasting dish and cook in the oven for 20 minutes, until collapsed and charred in places. Put in a blender or food processor with all the salsa ingredients except the sugar and the finely chopped green chilli, and blend. Add a large pinch of seasoning, then add a tsp of sugar at a time until the salsa is pleasantly sweet and sour. Add the finely chopped chilli, and stir.
Toss the prawns with some seasoning and the chilli flakes, and cook on the BBQ for 1 minute on each side, or until pink. (Alternatively, heat a large frying pan or griddle pan over a high heat. Drizzle the prawns with a little oil then fry or griddle, in batches, for 1 minute on each side until charred and pink.) Serve immediately with the salsa.