Heat the oven to 190°C/Gas mark 5.
Trim the leeks and slice into 2 cm lengths.
Cut the potatoes into 1 cm slices.
Put the oil and butter into a large, shallow roasting tin and put in the oven until the butter has melted.
Add the leeks and potatoes and turn them to coat them in oil and butter.
Coarsely crush the peppercorns and coriander seeds with a pestle and mortar and mix in the salt.
Sprinkle over the leeks and potatoes.
Cover tightly with foil and bake for 25 minutes.
Remove the foil (carefully!) and turn the leeks and potatoes over.
Return to the oven without the foil for a further 25 minutes.