The outside temperature is less relevant to your choice of tog than the temperature of your bedroom. It might be -5°C and blowing a gale outside, but if your heating is on full, you may not need anything warmer than a 9 tog duvet.
If you're concerned, invest in a simple thermometer to check the temperature of your bedroom. You should be aiming for a night-time temperature of between 16°C and 18°C. Anything warmer than 24°C will make you feel restless and unable to drop off. A temperature lower than 12°C will make it difficult to nod off.
For more information, see our seasonal guides.