Secondary glazing provides an excellent alternative to double glazing in cases where replacing the existing windows is not possible due to preservation orders, though as the seal is not as tight as double glazing, the reduction in heat loss through the window is not as much.
The idea is this, a second pane of glass on a light frame is attached to the inside of the existing frame. This second pane reduces heat loss in two ways. Firstly if creates a barrier between the warm inside air and the cold outside air, reducing convection losses. Secondly it traps a layer of air between two panes, reducing heat loss through conduction.
In cases where the window must be open-able, secondary glazing products are available which allow for this requirement. Options such as horizontal sliders, vertical sliders, and hinged units are available. Magnetic, clip on and screw on units which can be installed during colder months are a cheaper and more robust solution.