See if paying by direct debit or having dual fuel can save you anything.
Look closely at your fuel usage and see if there's anything you can manage better.
On top of ReadingMum's excellent advice: keep your feet off the floor when sitting down; if you have incandescent light bulbs, swap them for LEDs. Also, keep doors shut around the house. If you're cold at night, put an extra blanket on rather than the heating.
Also, and I know this sounds bizarre -- my partner told me about people complaining of an office being cold; the advice was that they should wear something that covered their neck (either as part of their top, or as a separate item like a thin scarf) -- and it worked!
Go back to the old school ways - wear a jumper and socks, snuggle in a blanket in front of the tv, make draught excluders for across doors (long tube filled with rice works well), keep curtains closed to retain heat, don't boil more water than you need for a cuppa, turn off lights when you leave the room, turn the thermostat down 1 degree, monitor usage by reading the meter regularly to avoid surprises.
When did that start? That people essentially felt entitled to dress like they were living in the Seychelles, rather than 52°N and whacked up their heating to compensate?