A heat cover doesn't create a fire seal as it has open sides or holes for ventilation. It is designed to keep enough space around the top of the fitting so insulation can be laid on top and the heat from the fitting is vented via the cover. If a fire seal is to be maintained a fire rated, can style downlight should be used. Only comment i have previously seen stated that they should still be fitted in case the lamps were changed back to halogens. Wasn't sure if there was a reg for it?Just been in my loft of new (old) house to discover insulation laid over the top of all the downlights. In for an itchy day tomorrow I think!!
Thanks