You are paying to heat the outside

Several answers point the same direction: warmed air is leaving faster than the heating can replace it. That is why the house can feel cold while the boiler is clearly working.

An open chimney, an unsealed loft hatch and door gaps together often account for more loss than any single wall. All three are afternoon jobs with materials costing less than a single month's difference on a bill.

Do the air leaks first. Insulation on a draughty house is money spent on the wrong problem.