For the rough puff pastry, sift the flour onto a work surface and make a well in the middle. Add the butter and salt to the well and work them together with your fingertips of one hand, gradually drawing the flour into the centre with the other hand. When the cubes of butter have become small pieces and the dough is grainy, gradually add 125ml/4½fl oz ice-cold water and mix until well combined.
Roll the pastry into a ball, wrap in cling film and chill in the fridge for 20 minutes.
Flour the work surface and roll out the pastry into a 40cm x 20cm/15in x 7½in rectangle.
Fold one third of the dough to the centre, then fold the other third on top. Push the edges of the pastry with the rolling pin to seal.
Rotate the pastry a quarter turn. Roll the pastry out again 40cm x 20cm/15in x 7½in rectangle and fold in the same way as above. Chill in the fridge for 30 minutes.
Repeat the rolling and folding process two more times to give four rolls and folds in total. Wrap the pastry in clingfilm and chill in the fridge for at least 30 minutes.
For the pie, preheat the oven to 200C/400F/Gas 6.
Heat a large lidded frying pan until hot, add the butter, leeks, shallots and garlic and sweat for 2-3 minutes, or until softened.
Add the whelks, clams and white wine, cover and cook for 2-3 minutes, or until all the shells have opened (discard any whelks or clams that do not open).
Remove the shellfish with a slotted spoon, remove the meat and set the meat aside.
Add the cream, bring to a simmer and cook until just thickened.
Return the clams and whelk meat to the pan, then add the smoked eel and heat through.
Season with salt and freshly ground black pepper, then add the chives and remove the pan from the heat.
Spoon the mixture into four individual pie dishes and brush the edges with the beaten egg.
Roll the pastry to a 5mm/¼in thickness and cut out four circles large enough to cover the pie dishes. Press the pastry on top of the filling and crimp around the edges to seal in the mixture.
Brush the pastry with the remaining beaten egg and pierce holes in the top to let the steam escape.
Put the pies into the oven on a baking sheet for 15-20 minutes, or until the pastry has risen and is crispy and golden-brown.