Put gooseberries into a pan with the water and castor sugar and simmer gently until turning yellow. Liquidise in food processor and then pass through nylon sieve. This way the fruit does not need topping and tailing first. Prepare the sauce by melting 1/2 oz. butter and adding the flour off the heat. Pour on slowly the gooseberry puree and stirring constantly bring to the boil. Finish by adding the rest of the butter, piece by piece, to give a nice glossy sheen. Serve hot.