Melt the butter in a saucepan, add the celery and cook gently for about 5 minutes, until softened.
Stir in the flour and cook gently for 1 minute, stirring all the time. Remove from the heat and gradually stir in the milk and stock. Bring to the boil, cover and simmer for about 15 minutes, until the celery is tender.
Gradually add the stilton and stir in until melted. Season to taste.