Melt butter in a soup pot over medium-low heat and cook leek for 8 minutes until very soft without browning. Stir in flour and cook for 2 minutes.
Add sherry and stir for 30 seconds. Gradually whisk in seafood stock, then add milk, Old Bay and white pepper. Simmer gently for 8 minutes, stirring frequently.
Remove from the heat and blend the base until completely smooth with an immersion blender. Stir in cream and return to low heat for 2 minutes.
Fold in the cooked lump crab without breaking it up. Heat gently for 3 to 5 minutes, stirring carefully, until the soup and crab reach 165°F without a hard boil.
Divide among 4 bowls, distributing the lump crab evenly through the pale bisque.