Eggs and leeks au gratin

(serves 4 people)
8 eggs
4 medium leeks, cleaned and chopped
3 oz strong Cheddar cheese (75 g), grated
1 tablespoon Parmesan cheese, grated
3 oz butter (75 g)
1 ½ or plain flour (40 g)
¾ pint milk (425 ml)
Cayenne pepper
Salt and freshly-milled black pepper
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