For the pancakes, place the flour, egg and sugar into a large bowl and add enough milk to make a batter the consistency of double cream. Add the parsley and a pinch of salt and mix well.
Heat the olive oil in a frying pan and add a spoonful of batter. Swirl the mixture around the pan to coat the base and cook over a medium heat. Flip or turn over with a fish slice and cook on the other side until golden-brown. Remove from the pan and set aside to keep warm. Repeat the process with the remaining batter to make a second pancake.
For the sauce, heat the butter in a small saucepan over a low heat and add the flour. Cook, stirring, for two minutes.
Remove from the heat and add the milk and cream gradually, whisking continuously.
Return the sauce to the heat, bring to the boil and simmer, stirring frequently, until the sauce thickens. Add the mustard and chives and season with salt and freshly ground black pepper.
For the filling, heat a griddle pan. Brush the leeks with olive oil and griddle until chargrilled and tender.
Fill each pancake with two of the leeks and the crumbled Stilton and roll up like a cigar. Heat a little oil in a frying pan and fry the filled pancakes, carefully turning, until crisp on all sides. If there is any excess oil, pat dry on kitchen paper.
To serve, place the filled pancakes onto plates and pour over the sauce. Garnish with the remaining leeks.