If the crabs are still alive, put them in a freezer for about 2 hours until they are rendered insensible, then sever their hind and front nerve centres (see this RSPCA guide for more details, http://bit.ly/1l0ZTAG).

Now heat the vegetable oil in a large heavy-based saucepan and fry the crabs over a high heat for about 5 minutes, breaking them up with the end of a rolling pin as they are cooking, until they change colour. Add the onion, leek, garlic, fennel seeds, thyme and bay leaf, and continue cooking for another 5 minutes or so, until the vegetables begin to colour.

Add the butter and stir well, then add the tomato purée and flour, stir well and cook for a minute or so over a low heat. Add the white wine, then slowly add the fish stock, stirring to avoid any lumps. Bring to the boil, season with salt and pepper, and simmer very gently for 1 hour.

Blend the shells a few at a time with a little liquid in a liquidiser or strong food processor, adding more liquid as they are blending, until smooth, then strain through a fine-meshed sieve.

Return to a clean pan, season if necessary, and bring to the boil. To serve, add the cream and some white crab meat if you wish. Adjust the seasoning again, if necessary, and stir well.