Installation of secondary glazing to Kemble village hall windows

Our village hall is used extensively by all members of the community, from the very young who attend the preschool, to the very old who attend a range of clubs. But it is a traditional building with large energy inefficient windows, and subsequently with rising energy prices we are finding it more and more expensive to heat the hall and to keep our more vulnerable members of the village comfortable whilst using the building.
We have insulated the roof during a recent refurbishment to make the building more energy efficient and reduce running costs, but the windows are large, single glazed and lose a significant amount of heat. We would like to install secondary glazing inside the property to make it warmer for our customers, more energy efficient and reduce our heating bills. We have chosen secondary glazing to achieve these goals so that it will not change the traditional appearance of the building externally.