Put the guavas in a preserving pan with ½ pt of cold water. Bring to a boil and then reduce the heat to low and cook for ½ an hour. Press the fruit through a sieve and place the pulp in a pan, together with the sugar, butter and lemon juice.
Simmer over very low heat for 2 hours, or until the mixture becomes very thick and dry. Stir from time to time.