The cavity must be closed, to prevent thermal looping and maximise the insulant efficiency of the air retained in the cavity. The traditional method to do this was to pack the cavity, shutter and pour a concrete "closer". This method was labour intensive and now most authorities would be happy to see the cavity closed with slates or tiles, bedded in mortar, with overlaps. Cavity Trays do a great product called the "Type J" cavity closer and support which gives you the "Rolls Royce" solution at a cost!