Heat the olive oil in a frying pan, add the bacon and fry for 3-4 minutes, or until crisp. Remove from the pan and set aside. Add the chopped onions and garlic to the pan and gently fry for 3-4 minutes, or until softened but not coloured.
Return the bacon to pan, stir in the flour slowly and mix until smooth to form a roux.
Drain the clams, reserving the liquid. Place the liquid in a measuring cup and add enough water to bring the measure up to 480ml/17fl oz. Gradually add the liquid to the pan with the bacon, stirring until it thickens.
Add the potatoes, chopped celery leaves, salt and freshly ground black pepper and bay leaf, bring to the boil and simmer until the potatoes are tender. If the chowder is too thick, thin down with some milk.
Add the clams and the butter and heat, stirring continuously, for 2-3 minutes but do not allow the mixture to boil. Pour into a bowl and serve.
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