Tariq, you have my full sympathy. Nobody warned Sue and me that the Portuguese coast, which looks so sunny in all the brochures, has houses built entirely for August. Our first January we wore our coats indoors and I seriously considered moving the bed into the kitchen because it had the oven.
What actually helped, roughly in order of how much difference it made:
- thick curtains, floor to ceiling, closed the moment the sun goes down
- draught strips on the doors and a proper door snake for that gap you describe (ours was big enough to lose a cat through)
- heating one room properly instead of the whole house half-heartedly
- a dehumidifier, which surprised me, but dry air feels warmer and it stopped the mould in the bedroom
None of it was expensive and the landlord was perfectly happy with all of it. We still get a big bill in January, but it's one I can open without sitting down first.