Old house, old windows. Ways to save on gas this winter?
You should have a thermography of your house to address where major leaks are (leaky windows and thermal bridges at least).
Best option, if you don't want to replace all windows, is, at least, to have the ones you already have, at least for north north/east side double-glazed.
Another effective, and not as expensive as double glazing, thing that help is insulating your roof from outside, if attic i living space, or just put some centimetres of insulation on attic's floor. Also you can go haunting for draughts and seal them with silicone.