You can save between 0.6 and nearly 2 tonnes CO₂e per year with solid wall insulation.

There are internal and external options. Internal insulation reduces the size of the rooms slightly but can be done on a DIY basis to reduce costs. External tends to be more expensive but doesn’t affect the internal size. Ask your mates about their experience.

Neither internal nor external insulation comes cheap but there are grants available in your area. It's certainly worth checking.