Heat the oil in a saucepan and gently fry the prawn heads, onion and garlic for 3-4 minutes.
Add the water and crumble in the stock cube. Add the tomato purée, lemon juice and beer and bring to a gentle simmer. Simmer for 4-5 minutes, until slightly reduced. Stir in the butter and season, to taste, with salt and freshly ground black pepper. Pass through a sieve into a clean bowl. Stir in the tarragon.
To serve, pour into a warm serving bowl and serve with crusty bread.