Now, in another pan, melt the butter and add the flour. Cook this, stirring to avoid browning, until it smells biscuity. Continue stirring vigorously (a whisk might be useful here) and add a couple of ladles of the liquor from the tripe pot. Once thoroughly mixed and smooth, return this mixture to the tripe. Stir in thoroughly and simmer for a further 15 minutes to allow the dish to thicken slightly. Adjust seasoning to taste and serve hot, using a slotted spoon, with mashed potato.