Heat the butter, oil and thyme in a large saucepan over medium heat and gently saute the leeks for 5 minutes. Add the potatoes and saute for a further 2 minutes. Add the pepper and stock and simmer for 30 minutes. Cool the soup to room temperature before blending.
Carefully puree the soup in your Russell Hobbs blender on medium-high in batches, until very smooth.
Return the soup to the saucepan, add cream and heat through.