Unless the chimney is being used for ventilation, it should be sealed to avoid warm air being sucked out of the room during winter. By filling the chimney with granular inert insulation, such as foamed glass pumice, the chimney void becomes part of the insulated wall. A new weather-shield chimney pot should be fitted to avoid large amounts of rain getting into the top.
Alternatively we can install an inflatable chimney pillow - preferably at loft level so that the chimney void is not cold - with new weather-shield chimney pot.
Further information is available in the Retrofit Pattern Book