Wash the mussels thoroughly. Heat in a dry drying pan until the shells open. Shell and beard the mussels. In a saucepan, melt butter, add flour and fry for 1 or 2 minutes. Remove from heat and stir in water, plus any liquid left from frying pan. Add salt and pepper, bring to the boil, cover and simmer for 10 minutes. Remove from heat. Stir in mussels and cream. Adjust seasoning and serve immediately.