To make cucumber relish, add the water and sugar to a small saucepan and put over low heat until sugar dissolves. Set aside to cool.
Use a mortar and pestle to ground peppercorns, then pound in the garlic and coriander roots. Add salt, grind for a couple of minutes, then add chilli and grind into a smooth paste.
Transfer to a bowl and stir through sugar water, fish sauce, lime juice, coriander leaves, kaffir lime leaves and cucumber. Set aside.
Prepare prawns one at a time (see Cook’s tip below). With a small, sharp knife, make an incision from just below the head down to tail without cutting all the way through, leaving head and tail intact and mid-section shell on. Prise open shell a little to devein the prawn. Repeat for remaining prawns.
Lay prawns in a flat dish and splash fish sauce over, leaving to marinate for about 10-15 minutes.
Preheat barbecue plate or griddle plate on medium heat. Brush each prawn with a little olive oil. Grill prawns on hot plate for about 4 minutes, basting with coconut cream. Turn over prawns and cool for another minute or so.
Pile prawns on steamed rice, top with cucumber relish, then garnish with shallots and coriander. Serve.