I rent and have avoided putting the heating on too much for years because of costs, preferring to find cheaper ways of staying warm.
Lots of good suggestions here but I looked to old fashioned ways as well; when my grandparents passed away I asked for their bed blankets which otherwise would have gone to recycling. Proper old fashioned bed blankets made from real wool - and a couple of acrylic ones with those little holes (name completely escapes me!)
They are brilliant for keeping warm at night - making up a bed the old fashioned way is so much warmer (still use a duvet as well) but with blankets and a top sheet. Putting blankets or throws under the duvet instead of on top is sooo much warmer!
Proper wool bed blankets are expensive to buy new but I've found a couple second hand on ebay that were really affordable.
I also use hot water bottles and invested in a couple of proper wool jumpers (instead of fleece) for layering in the winter.
Staying in all day when it's cold, the chill really gets to you, so go out for a walk and you'll feel much warmer for it you won't need to heat the house as much.
I put the heating on for an hour at a time about 3x during the day, once in the morning, once in the afternoon and before bed. It takes the chill out and with the other things I use its all I need.
I recently got an electric blanket for bed too, it was about £20 (you can put a single on a double bed too) and that's made nightimes really snug.
Keep doors closed, use curtains to retain heat, and draught excluder (easy to make yourself with old clothes you don't wear).
Don't forget wrist warmers too! I'll have a hot shower when I get home from work to help warm me up too if you can afford the few minutes of hot water it's a great idea (or sharing bath water with children).