Trim the leeks, cut off the green part and save it for another day. Halve the white part lengthways, then finely slice. Finely slice the shallots, then peel, deseed and cut the tomatoes into cubes. Roughly chop the parsley, including the stalks.
Melt the butter in a large pan over a low heat and gently fry the leeks, shallots, tomatoes and parsley until soft.
Season, then add the stock and 500ml of cider. Turn up the heat and allow the liquid to boil for 10 minutes, until it reduces a little.
Warm a large serving bowl or tureen.
Add the langoustines and cover with a lid for 3 minutes, then add the razor clams. Replace the lid and cook for a further 2 minutes.
Add the mussels and clams. Give it all a gentle stir, add another good splash of cider and replace the lid. Cook for a further 5 minutes, or until the mussels and clams have opened and the rest of the shellfish is cooked.
Using tongs and a slotted spoon, transfer the shellfish to the warmed serving bowl and put the sauce back on the heat.
Add the cream and tomato purée and a little more cider if you want more sauce. Stir well to combine, and pour the sauce over the shellfish in the bowl. Serve immediately with crusty bread.