5. Deal with floorboards, skirting and loft hatches
In a house with suspended timber floors, the gaps between boards and the gap where the boards meet the skirting can add up to more open area than every window in the room combined. Cold air rises from the void underneath, which is ventilated by design, so you are effectively sitting above a slow outdoor breeze.
For board gaps up to about 5 mm, a flexible acrylic gap filler applied with a caulking gun works and can be overpainted. For wider gaps, push in a foam or rubber gap-filler cord first, then caulk over it, or fit thin timber slivers if you want a period-correct finish. Do the perimeter gap at the skirting with a bead of flexible decorator’s caulk — that single line often makes the biggest felt difference in an old front room.
One firm rule: do not seal the air bricks or underfloor vents outside. They keep the joists dry. Sealing the floor above the void is correct; sealing the void itself invites rot.
Loft hatches are the other quiet loser. A hatch is a thin sheet of plywood in the ceiling of the warmest air in the house, and warm air collects right against it. Screw a strip of compression seal around the rebate so the hatch closes onto rubber, then glue 100 mm of rigid insulation board to the top face. Materials come to about $25, CA$35 or £20, and the hatch stops sweating on cold nights as a bonus.